Summary: "You found a loophole to suit your own needs? Why, Casey, I think I've been a bad influence on you."

Disclaimer: Don't own Life With Derek. 'Nuff said.

Author's Note: So, it's been quite some time since I last wrote a Life With Derek fic, but… I don't know. This idea randomly popped into my head a few days ago. Like, out of nowhere. Originally it was going to be Lizwin, but I decided it suited Dasey more, so… yeah. Anyway, I hope you guys enjoy.

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Letting Loose
By: Lady Azura

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"You've got our numbers, right?"

"Yes, Mom."

"And you'll call us if anything happens?"

"Yes, Mom."

"And no parties -"

"Mom…" Casey McDonald groaned and rolled her eyes, swivelling around in her chair and giving her mother a pointed look. "That was one time and it was Derek's fault."

"I know, I know," Nora sighed, trying to relax. "But just make sure it doesn't happen again, okay?"

"Okay, Mom." Casey assured her, standing up and making her way over to her mother.

Nora pulled her into a tight embrace. "We'll only be gone until Sunday. Marti's spending the weekend with Abby, so you don't have to worry about her, and Lizzie and Edwin can take care of themselves for the most part. They're at a sleepover right now, but I already told Derek to pick them up in the morning."

"So I guess I'm just babysitting Derek, then?" Casey remarked dryly.

Nora gave her daughter a sympathetic smile. "Just make sure he doesn't throw any parties or burn the house down. He's been surprisingly good lately — or at least as good as Derek can be — so I don't think you'll have anything to worry about."

Casey rolled her eyes. It was true that Derek had been unusually behaved over the last couple of weeks, but he was no doubt planning something. Derek Venturi couldn't go a day without making her miserable, and the fact that he'd gone weeks made her uneasy. All he did was stay cooped up in his room, blasting his music, and while that in and of itself was annoying, it wasn't nearly as bad as it could be. In fact, it was almost refreshing in a way, but Casey wasn't stupid. She refused to let her guard down. People didn't change overnight, least of all Derek. It was Derek. He had to be up to something, and whatever it was, it was something big. She just knew it.

"Nora!" George's voice jarred Casey from her thoughts, and she and Nora shared one last hug before Nora shouldered her purse and left the room.

Casey watched from her window as her mother and George pulled out of the driveway and disappeared down the road, before sighing. She combed her fingers through her hair and adjusted her headband before glancing around, her arms folded across her chest. It was impeccably clean, as usual; her homework was done, as usual; and the rest of the house was… relatively spotless. She pursed her lips, her brows knitting together as she debated what to do next.

She remembered Emily telling her about a party that weekend, and while there was no way she could actually throw one, her mother hadn't said anything about going to one. But did she really want to? She was bored, and she had already finished her homework and done her chores for the day. She deserved a break every once in a while, right? She could let loose, right? Have some fun? Derek did it all the time, after all.

Casey shook her head, not wanting to think about her step-brother. Thinking about him only made her angry.

Walking over to her closet, Casey began to rifle through, looking for something to wear. Eventually she pulled out a silver dress and examined it. Emily had convinced her to buy it on a whim a few months prior, but she hadn't yet worn it. It was short, barely coming down past her thighs, but Emily had insisted that she had great legs that she needed to flaunt more often. There was no back to it, and it dipped low enough to show a glimpse of cleavage, but that was all. She bit her lip before changing out of her normal clothes and into the dress. She then fixed her hair, giving it a bit more curl, and reapplied her makeup.

Once she was finished, she walked over to her mirror and took in her appearance. She couldn't help but smile. The dress wasn't nearly as skimpy as she'd originally thought — in fact, it was actually quite flattering, and she was glad Emily had made her buy it. Satisfied, she made her way to her dresser and opened the top drawer, fishing around for her favorite pair of underwear.

But it wasn't there.

Casey frowned, double-checking.

Still, nothing.

After checking for a third time, she let out a frustrated noise. "Ugh!"

Where was it?

She hadn't even worn then yet — there was no way she could've lost them!

Unless…

She paused, narrowing her eyes, her hands curling into fists.

"Derek." She hissed, before whipping around and storming out of her room.

She came to an abrupt halt in front of the door beside hers, already seething with anger, and knocked loudly.

"Derek!" She yelled. "Derek, you jerk, open up!"

There was no answer, but she could hear his music blaring.

"DER-EK!" Frustrated, Casey grabbed the doorknob and twisted it, throwing the door open.

Then she froze.

There, in front of her, was Derek, lounging on his bed with a bottle of liquor in hand. His eyes were wide and his mouth was agape; he looked like a deer caught in the headlights. She couldn't remember ever seeing him so stunned before.

Once the shock of what he was doing wore off, a triumphant smirk tugged at the corner of Casey's lips. Pressing her tongue against her cheek and crossing her arms, she shook her head slowly.

"You are so busted." She sneered. "I'm calling Mom and George."

With that, she turned to leave, but Derek was quicker, practically leaping off his bed and slamming his door shut.

"Wait… wait… don't." Was all he said.

"You're under age, Derek. I don't think our parents would approve." Casey told him, trying to get by, but Derek blocked her path.

"I'm not doing anything wrong, Casey. I'm not drinking and driving or doing anything stupid." Derek retorted, glaring at her.

"According to the law, you can't drink until you're nineteen." Casey said.

Derek scoffed. "Please. Everyone does it — even you, Case, so don't act all high and mighty. You drink that champagne crap sometimes. How is that any different?"

"I was at a wedding!"

"Your point?"

"It was an event, supervised by adults."

"So? This is an event. I like to call it 'Friday Fun with Derek'."

"That's not -"

"And I'm being responsible." Derek added, cutting her off.

"How is this responsible?" Casey demanded. "You're drinking under age, without mom or George here."

"Yeah, well, Nora said no parties, but judging by the eye shadow you're…" Derek started to counter, but trailed off when he finally took in her outfit. His brows threatened to vanish into his hairline and he swallowed hard, at a loss for words. After a few seconds, he managed to regain his composure and met her gaze once more. "Where are you going?"

"I'm meeting Emily at a party. Mom never said I couldn't go to one; just that we couldn't have one."

"You found a loophole to suit your own needs? Why, Casey, I think I've been a bad influence on you." Derek drawled. "But you're not going. Not dressed like that."

"Excuse me? Since when do you dictate what I can and can't do?" Casey snapped.

"Because I'm older. And if the guys at school see you in that, they'll…" He trailed off, his jaw tightening, before grabbing her wrist and dragging her over to his bed. He forced her to sit down.

"Derek -" Casey growled, trying to stand up, but Derek wouldn't let her.

Instead, he thrust the bottle he'd been holding earlier towards her.

"Try this. If you want to let loose, you can do it here." Derek said.

"I'm not going to drink that, Derek." Casey told him.

"Come on, Case." Derek prodded. "I thought you wanted to have fun. Trust me, this can be fun."

"I refuse to give into peer pressure." Casey declared.

"Just try it, Casey. If you don't like it, you can leave." Derek said, rolling his eyes.

Casey stared at the bottle for a few seconds before taking it from him. Tilting her head back, she took a swig, but immediately regretted it as tears sprung to her eyes and she hurriedly gave it back to him. She waved a hand in front of her mouth, forcing the clear liquid down before gasping for breath.

"It burns!" She cried.

"Here, try this instead." Derek said, reaching under his bed and pulling out another bottle. "It's not as strong."

Casey eyed the bottle suspiciously.

"It's fruity." Derek added. "You might like it. I promise."

Casey glared at him before taking it. After opening it, she took a small sip, swishing it around in her mouth and swallowing it. She licked her lips, savouring the sweet taste, oblivious to Derek's hard gaze as she did so.

"Better." She conceded, taking another sip.

Derek smirked. "So you'll stay here?"

Casey rolled her eyes.

"Fine. But just this once." She said.

.

.

.

About an hour later, Casey was doubled over, giggling uncontrollably as Derek stood in front of her, re-enacting an incident from his childhood.

"So then my dad was like, 'How could you fail recess?'"

Casey giggled harder. "How? How? It's recess! You can't fail recess — that's impossible!"

Derek laughed along with her before smirking cockily. "Didn't you know? I make the impossible possible."

Casey snorted, waving her hand dismissively. "You're so full of yourself…"

"Can't deny that." Derek replied with a shrug.

It was strange. Normally, she wouldn't find anything he said even the slightest bit amusing, but the alcohol was clearly clouding her judgment. Now every word that left Derek's mouth sent her into a fit of giggles, and she didn't care one bit. Derek was obviously enjoying this side of her too — the side that wasn't so uptight and, for lack of a better term, anal retentive. She could see it on his face; he seemed to genuinely be having fun. With her, his step-sister, the grade-grubbing keener. They were actually getting along for once.

Plopping down on the edge of his bed, Derek took another swig from his almost empty bottle. His face was slightly flushed and his eyes, fixated on the wall opposite him, were glazed over. Casey sat up, setting her own bottle of fruity deliciousness aside before moving to rest her chin on his shoulder. Derek stiffened instantly; her soft hair tickled his neck and he could smell her perfume. It fogged his senses and made it difficult to ignore the reaction between his legs.

"Derek…" Casey murmured, nuzzling the crook of his neck. "You smell really good…"

Derek swallowed hard, trying to calm his racing heart. Casey's arms slid around him, her hands massaging his chest through his shirt. He closed his eyes, his breathing becoming shallow as Casey's lips met the side of his neck.

"Derek…" She breathed, her hands dipping under his shirt, touching his hot skin with her cool fingertips. "Derek… I want you…"

Derek's eyes snapped open.

"W-what?" He managed to croak.

Casey immediately withdrew her hands, looking positively mischievous. "Gotcha."

Derek narrowed his eyes and before his step-sister could react, he was on top of her, pinning her to his bed. Casey gasped, staring up at him with wide blue eyes.

"D-Derek?" She squeaked.

Derek smirked.

"It's not very nice to tease." He said, jerking his hips.

A moan erupted from Casey, startling both of them. Derek's eyes darkened and he licked his lips, thrusting against her again. Another moan fell from Casey's lips.

"Derek, n-no-ohh…"

"What's wrong, Case?" Derek taunted, as he continued to roll his hips. "If I didn't know any better, I'd say you were turned on. Do I turn you on, Sis?"

Casey fought back another moan as a jolt of pleasure coursed through her, before wrapping her legs around Derek's waist and flipping them over so that she was on top. She could feel him against her, hard and pulsing, and bit back a whimper as she caught his gaze. Their eyes locked. Then Derek reached up, grabbing the back of her neck and pulling her down into a heated kiss.

She responded immediately — partly because of the alcohol, and partly because of years of unresolved sexual tension — and kissed him back with just as much fervor. Tongues tangled and teeth clashed as both teens tried to gain the upper hand, struggling for dominance. Eventually Derek won, rolling over so that he was once again on top. As they kissed soundly, he allowed his hands to wander, down her sides and to her legs and up under her dress, pausing to knead her thighs. Casey moaned into the kiss, lifting her hips, and Derek seized the opportunity to hike her dress up more, leaving it bunched around her waist. He finally broke the kiss for air, gazing down at the flush-faced girl beneath him.

Casey's lips were swollen, her hair dishevelled and her chest heaving with every breath she took. Her eyes were clouded with lust as she stared up at him, and he knew she wanted him just as badly as he wanted her. He watched her face as he slid his hands further up her thighs, letting his fingers ghost over her core through her underwear. Casey's breath hitched and she tilted her head back, but didn't object. Taking this as a good sign, Derek leaned down, sucking on the crook of her neck as he pushed her panties aside and touched her. She was slick against his fingers, and Casey's moans rang in his ears as he explored unfamiliar territory. He'd reached this stage with a few girls before, but this was different. This was Casey.

"God, I want you…" He groaned, before pulling his hand back.

Casey opened her mouth to protest, but Derek shook his head, giving her a smirk before sliding down the length of her body. He quickly discarded her underwear, tossing it aside and spreading her legs eagerly before burying his face between her thighs. Casey let out a shriek, grabbing his hair and gripping it tightly as his tongue swept over her, making her feel things she'd never felt before. Her heels dug into the mattress, her back arching as Derek put his mouth to good use.

"Ohh… oh God… Derek…" She moaned, her entire body quivering with pleasure.

Derek smirked against her, humming and swirling his tongue around before focusing his attention on her clit.

"DEREK!" Casey screamed as she came, fireworks exploding behind her eyes.

After helping her ride out her orgasm, Derek crawled back up her body.

"I love it when you scream my name." He murmured into her ear, making her shudder.

"Derek…" she breathed.

"Yeah?" He asked, nipping at her earlobe.

"I want… I want you to…"

"What do you want me to do, Case?" Derek prodded, running his tongue along the shell of her ear and making her squirm.

"I want you to… fuck me." Casey gasped.

Derek drew back, staring down at her in shock.

"Please," Casey begged, lifting her hips.

Derek groaned, any objections he had flying out the window. He quickly got rid of his shirt and pushed his jeans and boxers to his knees before grabbing her legs and hoisting them around his waist. His member throbbed with need and he searched Casey's face for any hesitation. He wasn't a complete asshole, after all. When he found none, he wasted no time entering her, making both teens cry out.

Her body felt like heaven, her tight heat clenching and unclenching around his length as she adjusted to him.

"Derek…" She moaned, once she was ready for him to continue.

He began to move within her, keeping a steady pace as he tried to keep himself from finishing too soon. He knew he wasn't going to last much longer, but he wanted Casey to finish first. He was feeling generous. Her legs tightened around his waist and she let out a guttural noise that nearly sent him over the edge, but he managed to control himself.

"Faster," she demanded, reaching up and clutching his shoulder. "Harder. Damn it, Derek, fuck me."

Gripping her hips, Derek picked up the pace of his thrusts, moving his hips faster. As the headboard thumped against the wall and the mattress squeaked beneath them, Casey lifted her hips to meet his thrusts.

"Yes… yes… ohh, God, there…" She chanted as he gave a particularly hard thrust.

"Say my name," Derek gasped as he neared his peak.

"No," Casey moaned defiantly.

"Say it!" Derek growled, hooking her legs over his shoulders and thrusting harder and faster.

The new angle sent Casey spiralling into another orgasm, screaming his name.

"DEREK!"

Derek smirked, just as a familiar wave of pleasure washed over him and he too came.

"Casey…" He breathed, burying his face in the crook of her neck.

They lay there in silence for a few minutes, basking in the afterglow of what they had just done, before Derek pulled out of her weakly and collapsed beside her.

"Jesus…" He panted, staring up at the ceiling.

"That was… amazing…" Casey breathed.

Derek nodded in agreement. "Yeah…"

There was a brief pause before Casey turned her head to look at him, a coy smile tugging at her lips.

"Up for another round?" She said, sitting up and swinging a leg over his hips.

Derek smirked, folding his arms behind his head.

"Your move."

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FIN

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… I have no idea where this idea came from, to be perfectly honest. All I know is that it popped into my head and I decided to write it. Hopefully you guys enjoyed.

I'd say it takes place sometime between season 2 and 3. It's AU, obviously, but… yeah.

It's been a long time since I've written Dasey, so I HOPE they were in character. Or at least as in character as possible, given the circumstances.

Anyway… I'd like to hear your guys' thoughts.

Please REVIEW and let me know!