Author's Note: I know, I know... another Dasey fic by moi lol. I just can't help myself, I keep getting these new fic ideas that just beg to be written. I've had this idea for awhile... I thought about what if Derek and Casey actually ARE together, but we just don't see it in the show because they are trying to fool everybody (their family, friends, the audience...). I know, very very improbable. BUT, it sparked the idea for this fic in my head. So here it is... it should end up being a few chapters long... if you guys all like it, that is. So, let me know what you think, and if you want me to continue it, I will :-)

Derek twitched a bit as he watched Sam take Casey's hand. He tried to act like it didn't bother him that they were together. Casey was his in secret. In public, though, Casey was with Sam, and Derek was with Emily. Nobody had a clue about the stepbrother and stepsister's hidden affair.

Casey had actually been the one to bring up appearance, surprisingly. Since everyone would freak out if they saw Casey and Derek together, they had to keep is hush-hush. And she was also the one to come up with the idea that they should stay with their "significant others." Sam and Casey had been going out for about 6 months, and Derek and Emily had been together for about 2 months.

He didn't know why he was even going out with Emily. Sure, she was fun sometimes. But he definitely didn't feel the way she obviously did. He wanted to break up with her. But it was hard enough watching Casey and Sam together. It would be even worse if he didn't have anyone himself.

He looked over at Emily as they sat on the bus. Then he realized she had been talking to him.

"Derek?" she looked at him expectantly.

He shook himself and forced a smirk. "Sorry, babe." He didn't even bother making an excuse.

Emily sighed, pursing her lips together and taking his arm. He didn't feel anything, unlike the sparks that he felt when Casey touched him. "What's the matter, honey? You've been spacing a lot lately."

Probably since he and Casey had started their torrid affair, a couple of weeks ago. She had consumed his every thought ever since.

"Uh, I just have a lot on my mind lately," he shrugged. "Hockey stuff, mostly."

"Aww," Emily said empathetically, stroking his arm. "You've been working hard, baby. All that practicing."

Some of the times he had made up these "practices," just so he could avoid going out with her. He didn't feel bad about it.

Derek nodded. He glanced over at Casey and Sam again, sitting a few rows away from them. Sam was talking to Casey and she was laughing, her nose wrinkling and smile bright. She looked really good today. It had been a school field trip to an art museum, and she had dressed up a bit for it. She was in a pair of tight black pants and a button down top, the outfit hugging all of her curves so perfectly. Her hair was half-off of her face, which Derek liked so he could see her deep blue eyes. She must have felt his eyes on her because her eyes shifted suddenly from Sam to Derek. She gave him a slight, almost imperceptible smile, and quickly looked back at Sam.

Derek gulped slightly, suddenly itching to get home and have Casey all to himself.

"…shopping?" Emily looked at him.

Shit. She had been talking again.

"Uh…yeah," Derek said, not even knowing what she had asked him.

Emily grinned brightly. "Great! I'll meet you there at five."

"Huh?" Derek's eyebrows furrowed. What had he gotten himself into?

"Meet me at the mall, silly," she smiled. "At five."

Derek inwardly kicked himself. And he had already used the "hockey practice" excuse three times this week. If he used it again, she might start to get suspicious.

"Fine," Derek wasn't happy about it at all. It meant less Casey Time. And he hated shopping.

Emily laid her head on his shoulder and he looked out the window, wondering how much longer it would take to get back to school.

As soon as they got back to school, Derek hurriedly left, not even bothering to go to his locker. He needed to leave quickly before Emily asked him to walk her home or something. He made it home before Casey, even though she usually got dropped off by Sam, who now had a car. Derek didn't think his best-friend was at all better than him because he could drive and Derek still had another month before he could get his license.

"Hi, Derek," Nora said to him as he walked into the kitchen to grab a soda. She was rolling some dough. "How was school?"

"Fine," he said simply and opened a can of Pepsi. He gulped half of it down quickly, the caffeine-filled liquid refreshing him from the boring day he had at the museum.

"How about the museum?" Nora asked as if she had read his mind.

"Boring," he answered.

Nora gave a short laugh. "Didn't you learn anything?"

"Yeah, that some famous artists really shouldn't be famous," he scoffed.

Nora laughed again, shaking her head at her stepson. Just then, Casey walked in. Her eyes and Derek's eyes met quickly.

"Casey, honey, how did you like the museum?" Nora asked her daughter.

Casey drew her eyes away from Derek's. "Umm…it was nice." She smiled. "The art work was beautiful."

"Two different opinions, as expected," Nora concluded. Then she said to them, "I'm baking cookies now. They should be done soon. Why don't you two go start on some homework and I'll call you when they're done?"

"Sounds good," Casey said quickly, heading for her bedroom. She gave Derek an expectant look.

Derek glanced at Nora, moving over in Casey's direction, "Uh, yeah, call us." He said shortly and the two of them went into the living room and up the stairs quickly.

As soon as they got to the top of the stairs, Casey pulled Derek to her, a big smile on her face.

"Hey," she said, eyes shining. Their mouths were millimeters apart.

Derek grinned. "Hey yourself." He crashed his lips upon hers.

Casey moaned happily, pulling Derek down the hall and into her bedroom. He kicked the door shut once they were in, and the two of them fell onto Casey's bed.

Derek closed his eyes, enjoying the feeling of Casey under him and his lips on hers. His tongue slid into her mouth and she moaned again, wrapping her legs around him, pulling him closer to her.

"I have to go out with Sam tonight," Casey gasped out to him as he moved his lips to her neck.

She sounded a little annoyed about it. Derek liked that.

"Yeah, I have to go out with Em," Derek said, his voice raspy against her soft skin. He pulled away and Casey gave him a little pout.

"We're not gonna have much time together tonight," she reached up and ran her hands through his messy brown hair.

He thought for a second. "Well, I'm gonna try to get out of staying too long with Emily. So you just come see me when you get home."

She smiled a bit, "Wow, that's actually a good idea. Who knew you were smart?" she teased.

He gave her a mock offended look and she giggled, wrapping her arms around him. "I'm just kidding," she murmured, pressing her lips back against his.

"Casey! Derek!" Nora suddenly called. "Come down for cookies!"

The two groaned synchronously.

Derek went in for Casey's lips again, but she pushed him away gently, laughing. "Derek, we don't want her coming up here. Plus I could use a sugar rush before my date."

Derek groaned and got up off of her. She sat up and fixed her hair before they headed towards the kitchen.

"So where are you and Sam going?" he asked casually.

"The movies and dinner," she replied. "It's for him to make up forgetting our sixth month anniversary." She rolled her eyes, clearly not over that.

Derek nodded. Even he had remembered that it was her and Sam's anniversary last week. I mean, how could he forget?

They went into the kitchen to find Nora, Marti, Edwin, and Lizzie all sitting at the kitchen table eating chocolate-chip cookies.

"Smerek! Have a cookie!" Marti offered, pointing at the plate in the middle of the table.

"I will, Smarti," he grinned at his little sister, taking a cookie and sitting down next to the bouncing girl.

Casey smiled to herself, watching Derek with Marti. He loved how the two were so close. Derek was so sweet with her.

"Are you going to have one, Case?" her own sister asked her.

Casey looked over at Lizzie, breaking out of her daze. Her cheeks pinked as Derek glanced up at her, knowing she had been staring at him. She nodded and sat down quickly next to her.

Derek gave her a slightly cocky smile, having caught her.

After their snack, Casey went upstairs to get ready for her date, and Derek played some video games with Edwin for awhile. Not too long after, the doorbell rang. Derek opened it, and, as expected, it was Sam.

"Hey, man," Derek said to him. He had been hanging out much less with him in the past few weeks.

"Hey," Sam replied. They did their usual best-friend male hand shake thing.

"Is Casey ready?" he asked, looking around the room as if she was there.

"I'll go check," Derek told him, gladly leaving the room. He jogged upstairs and didn't even bothering knocking on Casey's door. She was sitting at her make-up mirror, finishing up her lipstick. She had on a short skirt and a bra, having not wanted to risk getting make-up on her shirt because it was white.

Casey gasped when he came in. "Derek!" she breathed. "You have to knock."

"I do?" he leered at her. She looked great sitting there half-dressed. He stared at her white push-up bra in lust.

She sighed, seeing what he was staring at. She a slight smile graced her lips. "You've seen them before."

He had. Some of their make-out sessions got pretty hot and heavy. They hadn't gone all the way yet, but it had come pretty close. So seeing Casey in a bra was nothing new. But he enjoyed it all the same.

"So?" he winked. She smiled and went back to finishing her make-up. He walked over to her.

"Your date's here," he told her, trying not to seem bitter about it. He wrapped his arms around her from the back and began kissing the side of her neck.

"Mmm…" Casey closed her eyes briefly at the feeling.

He didn't want her to go on the date. Hell, he didn't want her to be dating Sam at all. He wanted her all to himself.

"Derek, I've got to go," she giggled, not making any real attempt to push him away, though. He pressed gentle kisses down her shoulder.

"Then go," he dared her to move as he gently moved his hands up her sides. He moved his lips back up to her neck and watched her lips part as he kissed his way down her spine from the nape of her neck. Her bare skin was soft and warm. Casey shivered.

He began tugging at her bra, and she laughed, finally swatting him away. "Get out of here!" she gave him a bright smile.

He grinned sheepishly.

She stood up and he watched her pull on her white top. He didn't like how tight and low-cut it was… for a date with Sam, anyway.

She grabbed her purse. "I'll see you tonight," she smiled, pressing a kiss to the corner of his mouth before exiting the room.

Derek stood there, disappointed. He sighed and looked over at the clock. It was almost 5:00. He should be heading to the mall, anyway.

He groaned and left the house. Emily ran over to him as soon as he got there. He was twenty minutes late, but she didn't say anything about it.

"Hey, baby!" she smiled and kissed him deeply. She wasn't a bad kisser, but didn't even compare to Casey.

Emily clung to his arm for the entire night. She made him go into the stupidest girly stores that he hated.

"Do you wanna go to a movie?" Emily asked, hugging him to her as they stood near a bench. There was a movie theatre in the mall.

Derek looked up at the titles of what was playing. There was a movie he wanted to see. It was a guy movie that he knew Emily would never want to see. "Only if I can pick the movie," he smirked down at her.

"Okay," she kissed him and dragged him to the theatre. As they were walking in, they literally bumped right in to Casey and Sam.

"Casey!" Emily exclaimed gleefully. "What are you doing here?"

Casey and Derek stared at each other for a split moment, surprised to see each other.

"Oh, we're on a date," Casey gave a half-smile up at her boyfriend, whose arm was around her waist.

Derek stared at Sam's arm, thinking of how he didn't deserve to be touching Casey. Feeling jealous, and hating that feeling, he pulled Emily closer to him.

"So are we," Emily grinned up at Derek. "Hey, we should double date sometime!"

Derek's eyes widened. "Uh…I don't think that's such a good idea."

"Yeah, we have totally different schedules, I don't think it would work," Casey added. She noticed Emily's hands all over Derek and tried not to look annoyed with that, which she was.

"I think it would be fun," Sam shrugged in his relaxed way.

"Great," Emily said. "How about tomorrow night?"

"I'm free," Sam nodded, and looked down at his girlfriend.

Casey gave a fake smile. "Yeah…sure…"

"Fine," Derek mumbled, knowing this was going to be a bad idea.

"Awesome!" Emily was just way too chipper. "Wouldn't it be easier if we just got together right after school?"

"Sure," Casey said through her teeth.

"Sounds cool," Sam said. He looked down at Casey again. "We should get going, Case. I made reservations for us."

Casey looked pleasantly surprised. Derek looked down at his feet.

"Have fun you two!" Emily called as they left. She squeezed Derek's arm. "They're so cute. Now, what are we going to see?"

It turns out that Derek should have left Emily pick the movie instead. He now had five half-moon shaped nail marks on his upper arm. She had screamed in his ear several times, too. Derek couldn't wait to drop her off at home.

"That was fun, Derek," Emily smiled as she stood at her door.

"Yeah," Derek said halfheartedly.

She kissed him and they said good night. Derek scuffed his sneakers against the pavement as he walked home in the dark.

He went right up to his bedroom and booted up the computer. There, he sat and waited for Casey. His Casey.