TITLE: The Sex Bra

SUMMARY: Derek makes an interesting discovery during his latest prank.

SPOILERS: general S3

PAIRING: Derek/Casey
RATING: T. I don't think it's graphic enough to warrant an M, but it is highly suggestive.
DISCLAIMER: I do not own Life with Derek. If I did, I probably would not be living in Borough Park in an apartment where whenever I turn on my microwave the lights go out.
AUTHOR'S NOTE: This idea came to me and would not shut up until I wrote it down.

The end may be a bit corny, but I happen to like corn.


The Sex Bra

He found it while pouring honey in her underwear drawer, and its appearance startled him so much that he dropped the bear-shaped bottle on his toe. Casey's underwear was something Derek had seen before – their habit of walking into each other's rooms unannounced had led to more than one encounter with his stepsister in various stages of undress. Not to mention that this wasn't the first time he had done something to her wardrobe – she'd had to shower three times before she got all of the itching powder residue off her skin.

But this was different.

Most of Casey's underwear was functional. She wasn't a slave to fashion trends like a vast majority of the girls he dated, which was refreshing, because nothing annoyed him more than a long spiel about why he needed to stop wearing his armbands, or why it was time to retire his favorite gray and pink polo shirt. So most of Casey's underthings were clearly for comfort and not aesthetic value. They were simple and cotton and almost always solid-colored – usually white. Casey McDonald, for whatever reason, did not go in for fancy lingerie.

So then how to explain the presence of this?

Derek held the offending object aloft with one finger, regarding it almost reverently, before taking it in both hands to examine it more closely. It was light blue and, Derek's breath hitched, completely sheer. The tops of the cups were lined with a frilly lace material that was also virtually see-through, and the straps were decorated with some kind of fancy floral stitching.

His heart started beating so rapidly that he feared it might burst in his chest. He had learned of this particular phenomenon while dating Kendra, and it had been reinforced once he started dating Sally. And judging by the snippets of conversation he often managed to overhear in the locker room, it wasn't just them, either. All women, Derek had discovered, possessed at least once fancy, usually expensive bra that was trotted out for special occasions. Like sex. After all, what was the purpose of buying a piece of clothing like that if no one were going to see it?

He had seen Kendra's 'sex bra' (red with black lace and something called a demi-cup, so her breasts practically popped out of it) – though not prior to having sex. He and Kendra had done everything else but had never made it that far because, people might be surprised to learn, he had not wanted to rush into things. There was a gigantic difference between messing around and sleeping around, and he did not really have any desire to be one of those guys. Besides, something had always felt wrong about being with Kendra. He hated that it was such a girl thing to feel, but Kendra had found it sweet – that he wanted to wait, not that being with her felt off.

He had also seen Sally's sex bra (dark blue with black flowers stitched on the front) – and that was prior to sex. And he wasn't about to tell anyone – because what would that do to his reputation – that it was his first time. Nor did he intend to tell anyone that it, as well as each time following, didn't feel right. The first time was awkward and uncomfortable, and Sally's repeated assurances that he would get better made him want to throw something. They'd been dating for a while, and Sally was starting to ask what was wrong, why he didn't want to sleep with her. And, hell, he was sixteen and she was gorgeous and half-naked, looking up at him with those eyes, and he let his hormones think for him. He felt sick afterwards. He tried to tell himself it was nerves, but deep down, he knew that wasn't the case.

It was just… He'd always secretly pictured himself losing his virginity to Casey. He'd thought about it quite a bit – especially after one of those encounters with her in her underwear – and he knew it had always been a possibility, considering how sexually charged their fights usually became. Derek had actually imagined that the two of them would move so seamlessly from arguing to sex that it would be over before either fully realized exactly what had happened.

Now, he thought grimly, that would never happen. Because he was still with Sally, whom he genuinely did like, and Casey surely had not bought this fancy bra for his benefit. And just what, exactly, did she need a sex bra for? There was no reason for her to have one. She shouldn't be having sex. She and Max had long since broken up, and he knew Max had gotten nowhere because the Orangesicle continually complained about all the 'wasted time' in the locker room. Derek had tried to shut him up once, and Particle Board's football buddies had nearly dislocated his jaw for his efforts.

He'd told the family it was from a vicious check into the boards. He was pretty sure everyone except Casey had believed it.

The very idea of some other guy seeing Casey in this made his blood boil. He knew he was being unfair – that he was allowed to have a sex life while Casey was not – but he didn't care. Casey McDonald was his, pure and simple, and he'd be damned if anyone but him got to see her in this bra.

The sound of the front door opening startled Derek out of his reverie. He replaced the bra and picked up the forgotten jar of honey, making sure that as much as possible spilled onto the light blue piece of fabric tucked in the corner of the drawer.

The first of his many attempts to see Casey in the sex bra involved him stealing all of her other bras and using them to stop up the bathtub.

"Derek!" she exclaimed, when she discovered her undergarments submerged under at least six inches of water. She stomped across the hall to her room, slamming the door for good measure. He counted off the seconds in his head, mentally envisioning her slipping the bra straps over her shoulders and reaching around to clasp it, before marching to her door and crashing through it unapologetically.

She screamed and made to slap him, and he was disappointed to see that she had already pulled on her shirt. He'd lost track of time during his fantasy. He barely noticed as her hand connected solidly with his chest, knocking him off balance so that he collapsed onto her bed.

Grumbling about insensitive jerks, Casey grabbed a sweater to wear over her tank top, and Derek was stunned to discover that – rather that opting to wear her special occasion bra – she had foregone a bra altogether. There was no flash of light blue visible underneath the black tank.

It was several minutes before Derek was physically able to move.

His next attempt required that he sneak into the girls' locker room. It wasn't exactly his first time doing so, but the school security guards had gotten suspicious after the second time, and so it wasn't quite as easy as it had been before.

He waited until the girls were out in the gymnasium before implementing his plan, which involved one of Edwin's smaller spy cameras that Derek had borrowed without his brother's knowledge. He'd gotten Ralph to snag some rubber cement from the art room and struggled to mount the tiny camera on the wall of Casey's gym locker. He knew she was wearing the sex bra today because he'd sneaked into her bedroom while she was eating breakfast to check. The special occasion bra was gone, and he'd also discovered that she owned the matching panties.

His normally deft fingers fumbled with the camera as an image of Casey wearing both pieces of lingerie popped into his mind unbidden. It was a struggle to keep his focus, and once he was finished, he hightailed it out of there.

The next time he saw Casey was later that day, in English, when she stormed up to his desk, wearing an expression that suggested she wanted nothing more than to rip off his arms and beat him with them. It was so utterly hot.

"I believe this is yours," she said, holding out the camera. He made no move to take it, because he wasn't about to admit that he'd been trying to catch a glimpse of his stepsister without her clothes on. Glaring, she dropped the camera on the ground and smashed it with the heel of her boot. With a flip of her hair, she turned and made her way to her desk, which she very ungracefully dropped into.

Derek waited until class had started to sweep the broken pieces under his desk in order to retrieve them. It was sick and wrong that her little display had turned him on like nothing else.

Casey had a date with Noel Covington, and she was, in true Casey fashion, freaking out about every last detail. Sally was over, snuggled up to Derek on the couch as they watched an old kung fu movie on television, but that didn't stop him from doing his best to torment Casey.

He could hear her shuffling around the in the bathroom, and even though he was in the living room (subtly trying to inch away from Sally – a man needed room to breathe, for the love of God), he knew that she was in front of the mirror, obsessing over her makeup. She was probably painstakingly applying the sparkly blue eye shadow she always wore when she was trying to impress a guy.

"It doesn't matter how much makeup you put on, Case!" He leaned his head over the back of the couch, in order to better bellow up the stairs. "He'll still be able to see your face!"

He was rewarded by the sound of Casey stomping down the stairs. He regarded her upside down as she stopped on the landing. "Derek!" Her lipstick was smeared, almost as though someone had bumped into her arm while she was putting it on, and she hadn't yet changed into the 'date outfit' that she and Sally had agonized over for ages before Derek stole his date back. "Do you mind? I'm trying to get ready for my date!"

He laughed. "I like your lipstick, Case. That's a good look for you."

Casey groaned and ran back upstairs. He was slightly disappointed that she hadn't opted to stay and continue their verbal sparring match. Their arguments were often the highlight of his day.

"Derek," Sally admonished, and Derek frowned a bit when it occurred to him that he much preferred the way Casey said his name, "be nice to your sister."

"Stepsister," he corrected automatically, turning up the volume.

He could almost hear Sally rolling her eyes, even over the badly dubbed soundtrack. "Be nice to Casey. She's really looking forward to this date."

Derek tensed automatically, but managed to relax his muscles when Sally frowned at him. "Oh, really?" He said it flippantly, trying to sound casual, like he didn't care. Like his stomach wasn't tying itself in knots at the thought of Casey dating that…that… Ugh. That.

Sally nodded, turning back to the movie. "She's been talking about it all week. She said she has something special planned."

When Casey came down the stairs a few minutes later, Derek had to make a concentrated effort not to swallow his tongue. She was wearing a soft pink tank top that clung to her curves and a short jean skirt that promised a peek if she leaned over too far. As far as Derek was concerned, an outfit like that was wasted on a guy like Noel.

Now, Derek would know what to do with an outfit like that.

"Wow," Derek breathed. Sally and Casey both shot him a look, so to cover, he said, "You look awful."

Casey glared at him, and Sally smacked him in the back of the head. "You look great, Case," Sally said reassuringly.

Casey furrowed her brow at Sally's use of her nickname but smiled at the compliment. "Thanks, Sally."

The doorbell rang, and Casey flounced over to let Noel in. His eyes bugged as he took in her ensemble, and Derek unconsciously tightened his arm around Sally's shoulders. Casey was grinning stupidly at Noel, ducking her head as though shy, and she was anything but shy. Derek didn't like this one bit – he didn't like the way she was gazing at him, or the way that he was practically leering at her.

Derek's heart momentarily stopped beating as Casey bent down to retrieve her purse, and he was able to see all the way down her shirt. Including, he realized with a jolt, the sex bra.

Fuck. Casey's 'something special' was sex. With Noel. Derek had the sudden desire to hit something. Preferably Noel.

She stood and caught him staring, but instead of looking away, as he normally would have done, he met her eyes almost defiantly. For one brief, glorious moment, they were locked in a gaze more intense and charged than any they'd had before. Both gave an almost imperceptible shiver, and then Casey broke the connection, turning to smile at Noel.

"Ready?" she asked, not waiting for a response as she brushed past him and out the door.

He was unable to think or talk about anything other than Casey and her date for the rest of the night. He and Sally ended up in a shouting match (the shouting was entirely on her end) because of, as she put it, his freakish obsession with Casey's love life. It was something she'd brought up before, but he hadn't realized she was quite this upset about it. He denied it, of course. He couldn't very well admit that she was right – he wasn't ready for this to be public knowledge because he wasn't really sure that 'this' was – but he got the feeling that she didn't believe him. Probably because she told him that she didn't believe him, and she had examples to back up her theory. Lots of examples. Irrefutable examples.

After what seemed like hours of talking, he managed to convince her that he was just concerned for Casey's welfare – he made up some shit about locker room talk – when his cell phone rang.

It was Casey.

"Derek?" There was something about the tone of her voice that instantly made him freeze. He didn't like how she sounded – scared. Broken.

"Casey?" In his peripheral vision, he saw Sally roll her eyes, but he ignored her. "What's wrong?"

Casey gave a choked sob, and Derek clenched his fists. If that jackass did something to her… But his train of thought was interrupted by Casey's soft, shattered, "Can you come pick me up?"

He didn't even need to think about it. He raced into the kitchen and grabbed the keys to the Prince off the counter. "Where are you?"

She gave him an intersection, not an address, and that's when it occurred to him that it was way too quiet on her end of the line. Wherever she was, she was alone, in unfamiliar territory, and it was getting late. He yanked his jacket off its hook by the door and was halfway outside when he remembered that Sally had no idea what was going on. He turned to tell her that he would be right back, but she was gone.

He didn't have time to think about the implications of that. He had to get Casey.

She was sitting on the curb underneath the stop sign on Sycamore with her knees pulled up to her chest, rocking back and forth. She didn't look up when he approached, so he killed the engine and got out of the car. He cautiously made his way towards her, and his breath hitched when he took in her appearance. Her hair was mussed, her makeup smeared, her clothing disheveled. She was holding up her tank top, because both straps had been snapped, revealing the light blue of her special occasion bra.

He was struck with the sudden urge to rip out Noel Covington's throat.

"Casey?" he whispered, hoping that what he was afraid happened hadn't happened. She looked to be unharmed, though, and knowing Casey, she would have put up one hell of a fight. He reached out a hand, but she flinched before he even made contact with her shoulder, so he withdrew it and sat down on the curb beside her. "Are you okay?"

She wiped her nose with the back of her hand and took a deep breath that turned into a strangled sob. "I thought I knew him."

Derek's fingers twitched. His heart was pounding. His left eye throbbed. "Did he – ?"

She shook her head emphatically. "No. He just – he was just…" She trailed off with a hiccough, hugging her arms more tightly around herself.

He had no idea what to do. He was afraid to touch her, he had no idea what to say, and, much as he wanted to, he couldn't leave Casey to go kick the bastard's ass. She solved his internal dilemma a moment later when she curled against him, burrowing into his neck and sobbing into his shirt. He didn't put his arms around her, didn't do anything to comfort her in any way, but she didn't seem to care. They sat like that for a long time, their legs tangled – him completely rigid, her crying uncontrollably – before her tears finally turned into sniffles and then went away completely.

"Want me to kick his ass?"

She gave him a small smile. "Maybe some other time. Can you just take me home?"

He nodded, standing, and held out a hand to help her to her feet.

They drove home in silence. Casey kept her eyes on the scenery, and Derek alternated between watching the road and checking on her. The radio was on, but for once, Casey didn't sing along. She just stared forlornly out the window. She didn't look at him until he pulled into the driveway, when she met his concerned gaze almost shyly.

"Thank you, Derek," she murmured. She leaned over the gearshift and gave him a quick kiss on the corner of his mouth. Then she opened the car door and plodded into the house.

Derek sat in stunned silence for a good five minutes before following her inside.

He tossed his jacket on the back of his chair and went upstairs. Soft music could be heard coming from Casey's room, and against his better judgment, that's where he went. For the first time since the McDonalds had moved in, he knocked on Casey's door. He didn't wait for her to give him permission, though, before peeking his head in. His mouth went dry as he realized that he had interrupted her changing for bed, and he quickly closed his eyes and shut the door. He barely got a glimpse of her in the sex bra before all saw was her stupid Swan Lake poster.

He blinked when he fully grasped what he had done. He had just passed up a prime opportunity to finally see Casey in that damn bra – the bra that haunted his dreams and occupied his every waking moment, the bra that he could not stop picturing her in every time he looked at her – but he couldn't do it now. Not when she was so vulnerable. He rested his forehead against the doorframe and took a deep breath. Something was happening; something was changing. Ordinarily, the idea of willingly facing a crying Casey would just not occur to him. But now, even though she had given him an out and he was in the clear, all he wanted to do was go in there and talk to her.

He told himself it was because he wanted to know exactly what had happened for when he beat the shit out of Noel.

She opened the door a few moments later, smiling softly at him. "Hey," she said.

"Hey," he mumbled, suddenly feeling very stupid and aware that he sounded lame. "I'm, uh… I'm going to bed. D'you need anything?"

She shook her head and became very interested in her fingers. "No, I'm okay. I hope I didn't ruin your date with Sally."

Derek pursed his lips. "You didn't. I managed to do that all on my own." He hunched slightly, trying to get her to look at him. "Don't worry about it. Are you sure you're okay?"

She shrugged and walked over to her bed. Derek took that as an invitation and followed, shutting the door quietly behind him. He wanted to sit on her bed, too, but he plopped into her desk chair instead. She didn't seem anxious to talk, but she wasn't making him leave, either, so he amused himself by trying to figure out her new password. He'd guessed her old one in about three days, but this one was taking some time. She was getting better at this.

"I thought he was different."

She'd said it so quietly that he barely heard her, but he glanced up from her computer. She was curled up on her bed, hugging that stupid blue monkey Max had given her for her last birthday.

"Case, what happened?"

She sighed and tightened her grip on Mister Muggles. "We were kissing," Derek's stomach clenched, and he was trying to decide if he really wanted to hear this, "and he wanted to go further. He started to take my shirt off and I – I told him I thought we were going too fast. And he said… He said…"

Derek had the distinct feeling that he was not going to like whatever Noel had said. "What did he say, Case?"

She sniffled, and his heart constricted. "He said that if I didn't want to, I wouldn't have worn that." She pointed to her hamper, and he saw the bra, caught on the corner and hanging over the side. It hit him then that she wasn't wearing a bra. He forced himself to concentrate on what she was saying, and not her chest, because this was important. "And I tried to explain to him that I hadn't worn it for him, I'd worn it for me."

Derek cocked an eyebrow, and his confusion must have been evident on his face, because Casey continued. "I bought it right after Max and I broke up. I needed to feel better about myself, and so I wear that whenever I need to feel…" She trailed off, blushing.

"Whenever you need to feel…?" Derek prompted.

She bit her lip, and his eyes flicked downwards. She really shouldn't do that. She had no idea how irresistible she looked when she did that. "Promise you won't make fun of me?"

He made an exasperated noise in the back of his throat, offended by the insinuation that he would laugh at her at a time like this. Yes, he had on occasion "kicked her when she's down" – in Nora's words – but not like this. Never like this. He wasn't that big of a creep. "Case, come on."

Her eyes were bright with tears that she was obviously trying to hold back. "Whenever I need to feel pretty."

He laughed. He couldn't help it, and he immediately clapped a hand over his mouth when he did. Casey glared at him, then threw Mister Muggles in his direction. Derek ducked, and the monkey bounced harmlessly off the wall.

"Derek!" She almost sounded normal, and he smiled. "You said you wouldn't make fun of me!"

"I'm sorry," he said, still chuckling, "but that's… I can't believe you need to do anything to feel pretty. Do you really not know how gorgeous you are?"

He froze, his eyes wide, and she gaped at him. He licked his lips, his brain scrambling to come up with something to say after that, a way to get him out of this, but he couldn't think of a damn thing. Afraid that he was just going to dig himself into a deeper hole, he stood abruptly, nearly knocking over her chair. "Well, it's late, and I'm tired, so, uh, I'm gonna go to bed. Okay? Okay. G'night, Case."

He didn't look back as he stumbled to his own room, and was only able to breathe when he was sprawled out on his bed. He scrubbed his hands over his face. The thought of Casey needing to do something to feel pretty… Because she'd broken up with Max and she was doubting herself and Derek didn't help her in the self-confidence department… And he was such a typical guy, thinking that the only reason a girl might wear a bra like that was because she wanted a guy to see it. His thoughts from earlier that night, about Noel not knowing what to do with an outfit like that... He felt sick to his stomach.

It was hours before he was able to fall asleep.

No one was all that surprised that Derek had no intention of going to his senior prom. He took any and every opportunity to poke fun at anyone and everyone who was wasting their money on what he referred to as a "glorified, overrated party". Naturally, most of his ammunition was reserved for Casey. She took it all in stride, which infuriated him, because her attention was focused on prom preparations for her and her date – some pretty boy named Andrew whom one of the cheerleaders had set her up with.

Nothing had come of his little admission that night. The next morning, she seemed more than willing to forget the whole night had ever happened, and he didn't want to do anything more to spook her, so he followed her lead. They went back to their normal behavior – sort of. Now, though, there was an underlying current of affection in almost every interaction they shared, and she very rarely got truly angry with him.

Sally broke up with him, apparently out of the blue. Derek, of course, knew the real reason, but he refused to tell Casey what went wrong. Sally, he supposed, wouldn't tell her either. But she kept asking. He fudged the truth as much as he could, not wanting to lie to her but not wanting her to know what happened, so he told her that he hadn't been what Sally expected. He was pretty sure that she didn't believe him, but she thankfully didn't press him for details.

Suffering in silence was something Derek Venturi just did not do, so he never missed a chance to give Casey grief about both the prom and her lame date. He felt it was all justified, because he had to listen to her yammer on about cummerbunds to match her dress and which flowers she wanted in her corsage. Her prom planning monopolized the dinner conversation for nearly a month until Derek started wearing earplugs to the table. It was much easier to avoid making eye contact with her if he couldn't hear what she was saying.

Two days before the prom, he was at his locker when he heard Casey's name. Turning, he saw Pretty Boy Andrew surrounded by a gaggle of his douchebag friends, and he was fairly certain they weren't discussing Casey's stellar GPA. Derek slammed his locker shut and made his way over to them.

Douchebag Number One, also known as Troy, said, "Come on, man. If she didn't give it up to Miller or Covington, and she was dating them, what makes you think she's going to give it up to you?"

Andrew scoffed and rolled his eyes. "It's prom. She's pretty much contractually obligated to put out. It's a rule."

Derek clenched his fists and struggled to remain calm as he strolled up to the boys. "Andrew."

"Venturi," Andrew said, raising his eyebrows. Derek couldn't blame him for being wary. Many of the male students had been steering clear of Derek ever since he broke Noel's nose.

"We need to have a little chat," Derek said. It took every ounce of willpower he possessed not to throw the jackass against the wall. "In private."

"Hey, whatever you need to say to me, you can say in front of them." Andrew gestured at his friends with his head.

Derek cocked an eyebrow, then lifted one shoulder in a shrug. His hand shot out and gripped Andrew by the throat. "I don't like the things you're saying about my stepsister, you ass. And I really don't like it when she cries. So don't make her cry. Do you understand me?"

Andrew could only nod, but that was good enough for Derek, who released him and walked away.

Two days later, Derek watched Casey sit on the couch and wait for Andrew to show up. She looked unbelievable. Her dress was some kind of red silk material that hugged her body in all the right places and flowed when she moved. Whatever she had done with her makeup made her eyes look bigger and brighter, and her lips looked luscious and inviting and entirely too kissable. Her hair was just begging to have fingers run through it.

The effect was ruined, however, by the fact that she was crying.

"Why isn't he here yet?" she asked Nora, who was patting her back soothingly.

Derek pretended not to hear this, keeping his eyes focused on the television. His stomach twisted guiltily, as it occurred to him that his warning to Andrew might have changed Pretty Boy's mind about escorting Casey to prom at all. He shifted uncomfortably in his recliner and tried to ignore the gnawing feeling in the pit of his stomach that he only ever seemed to feel for Casey.

An hour later, Casey's mascara had left trails down her cheeks, and Derek felt like he was going to throw up.

"Sweetie," Nora murmured, "I don't think he's coming."

"I don't understand," Casey sobbed. "What did I do wrong?"

Derek could take it no longer. He headed upstairs and rifled through his closet, searching for the dinner jacket his father had made him buy after he'd ruined the other one. He wouldn't match her dress, and Casey would probably fuss over the hair he could never seem to control yet acted like he spent forever styling it, but no way was he letting her not go to her senior prom.

Her sobs had tapered off by the time he came back downstairs, but knowing Casey, the waterworks could return at any time. He stood in front of her, nudging her knee with his. "Up and at 'em, Space Case. Let's go."

He was met with identical confused stares, though Nora's held a bit of skepticism. "What?" Casey breathed. "Where are we going?"

Derek rolled his eyes. As if his attire wasn't evidence enough. "I'm taking you to prom. Get up before I change my mind."

"But – but… You hate prom."

Neither chose to mention that he'd attended their junior prom – with Sally. Casey had been avoiding talking about Sally in Derek's presence for the past few weeks; she seemed to think talk of his ex-girlfriend would upset him. He just saw no point in bringing it up. It kind of ruined the gesture, made it look less altruistic and more recycled. Although he wasn't entirely sure that his taking Sally had been at all altruistic, considering she was just pissed at her boyfriend for not taking her to theirs.

"I do?" He furrowed his brow and tapped his lips with one finger. "What gave you that idea?"

The look on Casey's face was a mix between bewilderment and disbelief. She wiped at her eyes and smudged her mascara even more. "You've been complaining all month about – "

"Casey, I only do one non-Marti related good deed a year, do you want me to waste it on Edwin?"

She shook her head and got to her feet, stumbling as she tried to balance on her heels. Derek's hand on her elbow steadied her, and he couldn't help but smile as she scrambled to fix her makeup. He cleared his throat and wiped the smile off his face at Nora's suspicious glance. He nodded his head at his stepmother, not trusting himself to speak.

Nora insisted on taking pictures. "You two are actually getting along!" she'd exclaimed, snapping shot after shot of Derek and Casey doing nothing but standing in the living room. "It's an historic occasion!" Derek got through it by making faces until she finally gave up and let them go. Derek opened Casey's door for her, and she smiled at him, clearly surprised. A bright flash from the vicinity of the front porch told him that Nora had managed to sneak another picture. He rolled his eyes at her and climbed into the Prince. He was about to back out of the driveway when Casey gently placed her hand over his on the gearshift.

"Derek." She said his name in a tone of voice that she had never used with him before, and it made him blush. He tried to look at her and found he couldn't meet her gaze. "Why are you doing this?"

Derek took a deep breath and exhaled through pursed lips. He probably should not admit the real reason, but she'd unintentionally stumbled on the one way to get him to talk. "It's, uh, kind of my fault that Andrew didn't show up."

Casey looked at him, and her expression was so damn cute, and she didn't look mad – just confused. "What do you mean?"

He groaned. He didn't mind doing nice things for Casey as long as she didn't find out about them. Or confront him about them. And he wasn't so sure he should tell her this, because she'd read him the riot act after what had happened with Noel. Though he suspected that had more to do with the suspension he'd received and not Noel's broken nose. "He was talking shit about you, Case. Saying you were required to sleep with him because he was taking you to prom, and after what happened with Noel…" Derek swallowed. He didn't like remembering that night. He had hated seeing Casey like that. "So I may have threatened him a bit. You know, so he'd know that if he hurt you, he'd have to deal with me."

He expected her to yell. He expected her to scream. He fully anticipated that the vein in her neck would start throbbing. But he did not expect her to throw her arms around him. He was too stunned to do anything but return the hug. It was the first time he had ever really hugged her, and he didn't think he would be able to let her go. "That was so sweet of you, Derek," she whispered. Her breath was warm on his ear, and he shivered.

"Hey," he said, making no move to extricate himself from her embrace, "I don't do sweet."

"Oh, I think you do." She pulled away, and he caught a whiff of her perfume. She smelled like vanilla. He just wanted to eat her up. "Like calling my dad and telling him to come back."

Derek tugged at the collar of his shirt. "Well, that was because – "

Casey ignored him. "Or quitting your job because of the way the manager treated me."

"That doesn't count, I got my job – "

"Or when you gave up another week of your summer to help me out with Marti's camp."

"Okay, well, that was because you paid me."

Casey smiled and shook her head bemusedly. "Face it, Venturi. You're a nice guy."

"No, I'm not." He didn't know why he was fighting this. He wanted her to think he was a nice guy. She liked him when he was a nice guy. "I'm a skirt-chasing cad."

She cocked an eyebrow challengingly. "I'm wearing a skirt."

Derek gulped audibly. "So you are. Looks good on you, too. But I fail to see your point."

She sighed and sank against her seat. "Let's just go. We're late. We've already missed half the dance."

Derek cleared his throat, but his heart, which had lodged itself there as soon as Casey's arms went around him, refused to budge. "Derek Venturi is never late. Everyone else is just super early."

Casey rolled her eyes. The drive to school passed in silence, but Derek could have sworn that she kept looking at him. He resolutely kept his eyes on the road, gripping the steering wheel so tightly that his knuckles turned white.

The lot was nearly full, but he parked as close as possible and, on impulse, rushed over to the passenger side to help Casey out of the car. She smiled at him again, and his heartbeat accelerated automatically, then held out her hand. Derek blinked at it for a moment before grasping her hand firmly in his. They walked into the dance hand in hand.

The music didn't come to a screeching halt as they stepped through the entrance to the gym. They weren't met with a roomful of curious stares or loud whispers. In fact, very few people seemed aware of their arrival at all, but it was a momentous occasion for Derek, and as Casey led him across the dance floor, he marveled at how something as insignificant as her hand in his could make his entire existence somehow worthwhile. He experimentally ran his thumb across the back of Casey's hand and was pleased to see that she shivered at his touch.

"Casey!" Emily squealed, running up to them and breaking the moment. "You made it! I didn't think you were coming! I've been calling for an hour, but then I saw Andrew and – " She stopped abruptly as she caught sight of Derek, who raised his free hand in a pathetic attempt at a wave. "Casey, why are you here with Derek?"

"Andrew never showed," Casey said. She gave Derek's hand a light squeeze. "Derek offered to take me."

Emily narrowed her eyes. "For how much?"

Derek placed a hand over his heart in a gesture of mock hurt. "Why, Emily, I'm insulted at the insinuation that I don't have Spacey's best interests at heart."

"Like when you threatened her date?" Sheldon asked, appearing, as usual, out of nowhere. Derek started at Schlepper's sudden arrival – he'd always meant to put a bell around that kid's neck.

Derek shrugged nonchalantly. "Hey, if a guy is really interested in a girl, he won't let himself be intimidated. All I told him was that he'd better not hurt her. And what did he do? Hurt her. Guy's a prick. Case is better off without him. Next topic, please."

Emily opened her mouth, and Derek feared they be analyzing his motivations all night, but before she could speak, Sam stumbled out from between two wildly gesticulating cheerleaders. "D, I didn't think you were coming, man!"

Sheldon nodded at Derek and Casey's still-clasped hands. "He brought Casey."

Sam raised an eyebrow, and Derek cleared his throat, suddenly uncomfortable. He did not, however, release Casey's hand. "She begged me," Derek said. "There were tears. Not pretty."

Casey rolled her eyes, but she was smiling. "Men are so easy," she stage-whispered to Emily. "Toss a few tears their way and they fold like card tables."

Derek gave a loud, fake gasp and turned to Sheldon. "They can fake tears? Is that legal?"

Sheldon reached for Emily's hand, lightly kissing her wrist. "Hey, when she looks this good, everything is legal."

Derek privately agreed, doing his best to stare at Casey without everyone knowing that he was staring at her. Then a song came on that he vaguely recognized, and Casey was bouncing up and down. "I love this song!" she squealed, grabbing his hand and pulling him onto the dance floor.

There may have been whispers. There may have been stares. If there were, Derek was unaware of them. The only thing he noticed was Casey – that she was smiling wider than he'd ever seen her smile, that she looked amazing in her dress, and that the way she danced, the way she moved against him, took his breath away. He could barely stand to be away from her, kept at least one hand on her all night, be it a light touch on the small of her back as he led them to their table or his firm grip on her waist as they danced. He felt giddy, drunk, high, yet the only thing he'd had all night was soda.

He wondered when this had happened – when Casey had become his drug, his addiction, the one thing he needed in order to function like a normal human being. Maybe it had always been like that and he'd just been too deep in denial to acknowledge it. But now he was fairly certain that he would not be able to get through the day without his Casey fix. And he really didn't think he wanted to.

He couldn't take his eyes off her. She was enchanting. She was his. And he had a feeling that she knew that.

The night passed quickly, and soon – too soon – Derek and Casey were walking to their front door. Derek was considering attending an after-prom party, but Casey was tired and wanted to come home. He intended to make sure she got in okay and then join Sam at one of the parties, which was being thrown by one of the guys on the hockey team, but there was an odd tugging in his gut that suggested he stick around for a while. Maybe it was because this whole situation reeked of a date and not a stepbrother doing something nice for his stepsister. True, he hadn't taken her out to dinner, he hadn't brought her flowers, he hadn't even paid for her prom tickets, but they'd barely been out of each other's company since leaving the house earlier that night, and Derek couldn't be certain, but he was fairly positive that there'd been a shift in their relationship. They'd never developed the brother-sister bond that Edwin and Lizzie had, because he'd never wanted her to be his sister. He'd always wanted her in another capacity. He got the feeling Casey sensed it, too, and that's what was making her look at him the way she was. And this was far different from last year's prom, even though it had started similarly, because at the end of the night, it was still just the two of them. It was always just the two of them anymore.

They stood on the front porch, though it would have made more sense for them to just go inside, which made it feel even more like a date. Derek jammed his hands into his pockets and eyed Casey warily. She was playing with her fingers, shifting her weight nervously. He raised his eyebrows.

"I had a great time tonight, Derek," she murmured, not making eye contact. She kept her gaze determinedly on a spot just over his left shoulder.

He coughed and went for casual. "Surprisingly, I did, too. And here I thought I'd die of boredom being forced to spend so much time in your company."

She rolled her eyes, but the corners of her lips tugged upwards and he knew she was trying not to smile, and she was reaching out – probably to punch his shoulder – and before he really knew what he was doing, he had a firm grip on her wrist and had pulled her flush against him and lowered his mouth to hers.

She stiffened, inhaling sharply through her nose, and he froze, afraid that he had misinterpreted the signals, that he had completely misread the situation. But her reluctance lasted only half a heartbeat before she was kissing him back with the same reckless abandon with which he was kissing her.

Groping blindly for the handle of the front door, he stumbled into the entranceway, Casey following soon after, her hands gripping fistfuls of his hair almost painfully as she made sure that their lips remained fused. He kicked the door shut, and she pushed him up against it, reaching around him to turn the lock. He put his hands on her shoulders, slowly dragging them down her arms, taking the straps of her dress with them, and finally broke the kiss because he was going to pass out from lack of oxygen.

Casey took a half step back, breathing heavily, her eyes locked on his. Her pupils had dilated, her cheeks were flushed, her lips were swollen, and he was almost drawn back in – oxygen be damned – when the light suddenly turned on, and Derek found himself staring at his dad.

His cheeks burned, and he knew that he was most likely a furious shade of red, but he didn't attempt to straighten his appearance because he knew that would only draw his dad's attention to it. It was best to just act like he hadn't been doing anything wrong. "Hey, Dad."

"George," Casey said, in a voice that sounded completely unlike her own, but he couldn't quite pinpoint why.

His dad, thankfully, did not seem to have noticed what they'd just been doing. "You guys just get home?"

Derek nodded, rubbing the back of his neck. "Uh, yeah. Spacey didn't feel like hitting any of the after-parties, and I'm kind of beat myself. All that dancing."

"You danced?" his dad said, laughing. "Casey, how much did you have to pay him?"

Casey forced a laugh. "My first born." She yawned then, very theatrically, using her arms and everything. "Well, I'm tired. Good night." She practically bolted up the stairs.

"Did I say something?"

Derek shook his head, inching towards the stairs himself. "Nah, you know Space Case. She's always weird like that. I think I'm going to head up to bed myself. Night, Dad."

He barely heard his dad's "good night" as he tromped up the stairs towards his room. Rather than going right in, though, he stood in the hallway, staring at Casey's closed door, having a mental debate with himself as to whether or not he should go in. They could probably ignore it like last time, pretend that it never happened, but he didn't want to. She had kissed him back – that was no hallucination on his part. But on the other hand, he did not really feel up to having the necessary conversation this late, so maybe it was best to wait until the next morning, when they'd both had a decent night's sleep.

Yeah, like he was going to get any sleep.

Shaking his head in disgust, he opened his door…

…and came face to face with Casey, kneeling on his bed, wearing only her special occasion bra and the matching panties.

He knew, deep down, that it was impolite to stare, but he could only gape at her, his eyes wide, his jaw slack, as he took in the sight before him – his stepsister, sitting in her underwear on his bed. His first thought, once he was capable of forming coherent ones, was that his imagination had definitely not done her justice. She was curvy in all the right places, and slender in all the others, and that bra was doing amazing things to her breasts. He must have been a very good person in a previous life to be rewarded so spectacularly in this one.

He probably should have said something classy and romantic, but that just wasn't his style. He probably should have said something – anything at all – because they probably should talk about this. It was a pretty big step they were taking. It was way different from kissing on the front stoop. But he said nothing. He pounced on her instead, pinning her underneath him. She giggled – he never thought he'd ever be able to make Casey giggle – and smiled up at him, her eyes wide and trusting, and he wondered what the hell he'd done in his past life to deserve this.

But the memory of that night was too raw in his mind, her words about how she just wore that bra to feel pretty replayed in his head. He closed his eyes, lowering his forehead to hers, and took a deep breath.

"Are you sure?" he asked quietly, hoping he would have the strength to respect her wishes if she said no. "Because I've gotta tell you, if this is a joke, it really isn't funny."

He felt, rather than saw, her nod. "I'm sure." He opened his eyes. "It's prom, after all. I'm contractually obligated to sleep with you."

Her eyes twinkled, but her words rung in his ears. He pulled back, desperate to make her understand that he was interested in more than that, that it wasn't just one night for him. "Case, that isn't why – "

"I know," she said quickly, grabbing a fistful of his shirt when he made to roll off of her. "I know that. I want you, Derek. It's always been you."

Hell. Good enough for him.

Afterwards, they sat entwined on his bed, watching a DVD on his laptop. He reclined against the headboard with his arms securely around Casey, seated between his outstretched legs. His chin rested on her shoulder, and he entertained himself by blowing lightly on her ear. She laughed breathlessly, trying to squirm out of his grasp, but he held tight. He wasn't about to let her go now. They wrestled for a while – a much more enjoyable activity than it used to be, given their current state of undress – before Derek once again had Casey pinned to the bed. He was smiling, and she was giggling, and he wondered if it were possible for his heart to burst from happiness.

He rubbed his nose against hers. Something welled in his throat. He thought it might be sap. He was in danger of choking on it. "I thought you said you wore that bra for yourself. And not for a guy."

She blinked, her blue eyes registering confusion momentarily before comprehension seemed to dawn. "I do. But, sheesh, Derek, you don't buy lingerie like that unless you expect someone to see it."

He gave a bark of laughter and hastily glanced at his door to ensure that the lock was still secure. "Oh, really? And just who, Miss Casey, did you expect would be seeing that?"

She sniffed and said with faux haughtiness, "Well, my annoying stepbrother does constantly barge into my room without knocking."

He licked his lips. "Hell, if I'd known you had underwear like that, you'd better believe I would have barged in a lot more."

She gave an over exaggerated sigh. "Damn. I guess everything has consequences."

Then she surged forward and latched her lips onto his. There was very little talk after that.