"Why does your roommate hate me?" Tina whines, tugging on Derek's shirt.

"She doesn't hate you, babe," Derek mutters, attention focused on the hockey re-run playing on the TV.

"Derek," Tina stands up and blocks Derek's view, arms on her hips. "Look at me."

Derek rubs his eyes tiredly, resigning himself to the fact that he definitely won't be watching the rest of the game.

"What's up?"

"Don't 'what's up' me!" Tina stomps her foot and glares. "I want to know why Casey hates me!"

"Babe, for the last time – "

"Don't you dare say she doesn't hate me, Derek Venturi! When I come over, she's never home; the rare times she is here she either leaves before I can so much as say hello or locks herself in her room; she hasn't even formally introduced herself to me for fucks sake!" Tina ticks of the reasons on her fingers then stares at Derek expectantly. "Well?"

"I haven't noticed," Derek replies casually, "she's always been a weird one."

"You're fucking unbelievable," Tina mutters, grabbing her coat.

"Tina, where are you going?" Derek calls as she walks away.

"Home. Call me when you pull your head out of your ass." She slams the door shut and Derek flops his head onto the back of the couch.

Their month-long relationship had been going just fine, Tina was relatively low-maintenance, the sex was decent, and he actually enjoyed her company for the most part. Then she got the idea into her head that Casey hated her and wouldn't let it go. It's not that Derek didn't notice his step-sister avoiding the apartment, stumbling in during the early hours of the morning – if she even came home at all – dark circles under her eyes and an ever-shrinking frame. Of course he noticed. Just like he noticed that they've only exchanged maybe five words the past month and that none of his teammates have heard from her either. Maybe she's talked to Leah, but Derek wouldn't know, the redhead avoids him every time they cross paths. So yes, Derek knows that Casey hates Tina, but how is he supposed to explain that to his girlfriend? How is he supposed to explain the five years of hatred for each other's current fling? How is he supposed to explain the history of fake support, scathing comments, and inevitable sabotage followed by some sort of peace offering? How is he supposed to explain that they're just…Derek and Casey…without explaining how badly he misses the fucked-up mess that they always get themselves into? He can't. And he can only avoid the Casey Guilt if Tina stops bringing it up. So Derek does what he normally does, he avoids the situation and turns his attention back to the TV.

xxxx

After a week of ignoring him, and most likely realizing that he won't cave, Tina gives Derek a call. She says something along the lines of "it's not your responsibility to know how Casey feels" and "I'm sorry for blowing it out of proportion," but Derek isn't really paying attention. Because, of course, the day Tina decides to end her silent treatment is the day Casey finally shows back up. She's currently in the kitchen, reaching into the fridge and retrieving a water bottle. Derek can't look away, if he blinks, he might miss her.

"Anyway, I'll be over soon!" Tina exclaims, bringing Derek back to attention.

"Wait, now's not a good – " Tina hangs up. "Time." Derek mumbles.

Pocketing his phone, Derek goes into the kitchen and leans on the counter across from Casey. She ignores him and he takes the opportunity to actually look at her. Baggy clothes hang off her body loosely, her hair is pulled up into a disheveled bun…high school Casey would have never let herself get this bad.

"You look like shit." It's not what Derek wants to say, but it comes out anyway.

Casey rolls her eyes. He watches as she twists the cap back onto her water bottle and his eyes follow her as she starts to walk away.

"So you're just gonna keep running?" He didn't mean to say that either, but it's out now and he can only hope she'll take the bait. She continues to walk. "I'll take that as a yes?" Derek knows he's provoking her and can't help but grin as he watches her turn around.

"How dare you?" Her dull eyes ignite with that spark of indignation reserved solely for him. "How dare you accuse me of running." She gets closer and the air hums with the crackling electricity of Casey McDonald letting her anger loose. "You," she stops in front of Derek, voice quiet and venomous, "are the one who ran away." She jabs his chest. "You are the one who left me." She hits his chest this time. "You abandoned me, you avoided me without so much as an explanation," she hits his chest again, and again, "then shoved your new whatever-she-is in my face to make yourself feel better. You're a fucking coward, Derek Venturi, and I – "

Derek reaches out and pulls her towards him, ending her rant. She looks up at him and his hands are on her hips and her hands are tugging his jacket forward and he's leaning down and…

Someone knocks on the door and he moves away from her like she's on fire. Casey looks at him with disappointment and shakes her head.

"I'm done, Derek."

"Casey, don't…" there's another knock on the door.

"You better get that."

Derek groans. College wasn't supposed to be like this. It was supposed to easier here, not harder. There's another knock on the door and he storms over to swing it open.

"What?!" He snaps.

Tina looks up at him, eyes wide with shock.

"I, um, I wanted to see you," she stutters.

"If you had listened to me for once you would have realized that it's not a good time," Derek spits out.

Tina looks down, noticing a new pair of shoes in the entry way.

"It's Casey, isn't it?" She asks softly.

"What?"

"This," Tina gestures between Derek and herself, "can't work, can it? Not with Casey around."

"Tina," Derek runs his hand through his hair, "I'm sorry I snapped at you, I'm just a little stressed out right now. Casey and I, there's no, I mean I never…"

"Don't worry," Tina smiles sadly up at Derek, "I don't think anything happened. There's just no room for me here. Whatever the two of you have, there's no space for anyone else."

Derek stares at Tina, speechless, for once.

"We had a fun run, Derek," Tina pats his arm, "I'll see you around."

Derek heads back inside and promptly finds himself in front of Casey's door. He takes a deep breath. He's not fifteen anymore, he's not in London, and Casey has never been family to him. She's always been so much more.

This is his last chance, he knows it. So before he has a chance to overthink it, he twists the door handle and barges into her room.

"Der-ek!" Casey shrieks, throwing a pillow at him with frightening accuracy, "get out!"

"Nope." He closes the door behind him and leans back against it.

"I'm sorry."

"If you were sorry, you'd get out of my room," Casey scowls at him.

"Tina's gone. For good."

"And I care because?"

Derek crosses the room to sit next to Casey on her bed and she backs away instinctively.

"Dammit, Princess, don't be difficult," Derek says exasperatedly.

Casey stares at him in stubborn silence and he sighs heavily.

"Fuck! You know I'm not good at this shit, Case." Derek looks down at the comforter then looks back up "It's always been you, Casey. I'm done pretending. It's fine if you don't," he swallows hard, "want anything to do with me."

Casey stays silent.

"I'll be around if you change your mind," Derek smiles awkwardly at Casey and she looks back at him with a blank expression.

"Right then," Derek clears his throat, "I'm going to leave now."

xxxx

Derek finally caves into how utterly exhausted he is and heads to his room. No more than ten minutes after he lays down in bed, he hears his door creak open, followed by his covers being lifted and the bed dipping down slightly next to him.

"You're a fucking dick, you know that?" Casey mumbles into his back as she drapes her arm over his torso.

Derek grasps her hand gently.

"Yeah, I know."