Derek knows Casey the best. He would wager that he knows her even better than her mother. Derek has seen the best in Casey, the side that she puts forward and wants everyone else to see. The perfectionist. However, he has also seen Real!Casey, the one with stray hairs, no makeup, the Casey who cries, wears sweatpants and wrestles with him for the remote. Derek doesn't have a favorite version because they are sworn enemies, but if he had to pick, the second version of Casey would be his favorite.

It never made much sense to Derek why all the guys she dated had trouble figuring her out. Derek knew her. If he could figure it out, they should be able to. Granted, Derek learned to understand the way her mind worked so that he could prank her more effectively.

That said, Derek doesn't always get Casey. Sometimes she does or says something, and he can't quite figure out where her thoughts lay, or which mental process leads her to where she ends up.

The perfect example is on their way to University. It is the first semester, and they're in the car. Derek is driving with Casey beside him, going through a list of all the things that Derek is Not Allowed to Do.

"... and if we wind up having a class together, sit as far away from me as possible."

"Casey," Derek says, exasperated, "I know you. You're going to be in the middle of the front row. What makes you think I'd try and sit with you?"

She is primly perched in the passenger seat of the Prince, checking off rules in her notebook. "Because," she states matter-of-factly. "You'll likely see it as an opportunity to get through a class without having to do the work, and you live to annoy me. Naturally, you'll sit by me so you can ask for my notes and try to distract me."

Derek snorts. "I can ask you for notes after class. I'll sit well away from you. Scout's honor." It doesn't sting, not even a little bit that she's so insistent that they stay far away from one another at Queen's. After all that he went through to find out which school she was picking.

"Which brings me to rule fifteen subsection A: if we do have a class together, you may not ask to borrow my notes unless you were absent due to illness." She considers that and sniffs, "and even then."

"Casey," his tone is a little sharper than he intends, but the last thirty minutes have been a dissertation on staying one hundred yards away from her, not that he's offended or anything. "I get it. Leave you alone; the same applies to you. If you feel that strongly about it, e-mail me your list of rules, and I'll do the same." It was unlikely that Derek would ever bother to write up a list of rules, but he might be persuaded to do it if only to annoy Casey. Sending her a list of things she's not allowed to do is a surefire way to manage that. "You don't want me around. You've made your point. Can we not spend the rest of the drive going over this?"

She starts to say something, and his hands tighten on the steering wheel. If she finds another way to reiterate avoiding each other, Derek might lose it. Neither of them will make it to school because they'll have died in a tragic car accident.

"Okay."

"Okay?"

Casey nods, and they spend the rest of the drive arguing over the radio. The knot that had started in the pit of his stomach at Rule Number One starts to loosen.

The first time he sees Casey after they've moved into their respective dorms is the weekend after orientation, right before they're supposed to start classes. She's walking arm in arm with a girl, and they're rooted in conversation. He stops for a moment, trying to figure out his next course of action.

Some point after Casey and the rest of the McDonalds move in, a large part of Derek's actions are motivated by being the antithesis of Casey's desires or simply designed to annoy her, but that was when he thought that Casey might enjoy the way they're frequently at odds. The fact that she so desperately wanted a start without him had hit home on that drive.

Not that he's in pain over it or anything. That knot in his stomach? It was the meatloaf he had for dinner in the dining hall last night.

So he's standing there thinking of whether he should go and annoy her or simply move on when she looks up at him. Their eyes meet. Casey gives him a small smile, but there's a look in her eyes. He can't quite place what it is, but he thinks it might be panic.

He keeps walking, not even acknowledging that they had locked eyes. The meatloaf in his stomach twists a little tighter.

They do end up having a class together. It's an English class, of course, and one of the smaller ones. There were ten students in total and only twelve desks. It's no surprise that Casey is sitting at the front. When Derek walks in, seconds before class starts, their eyes meet.

There's a seat available in the front next to Casey, and there's one available further down the row next to a preppy looking blonde.

Derek is annoyed at the momentary hesitation he feels before heading to the second option. It's not that he wants to sit next to Casey - of course he doesn't - but she had a point about getting notes from her. He slides into the seat and the blonde smiles at him. When Derek glances at Casey, she's staring straight ahead with her fingers gripped tightly around her pen.

The whole seat situation turns out to be pointless because, after attendance, their professor has them rearrange their desks into a circle. Derek winds up next to Casey anyway.

He's acutely aware that Casey is angry (and very likely at him), but this time, he can't figure out why. This time, he doesn't get it; Derek doesn't get her. It's an unusual feeling.

"You're following her rules?" Edwin asks in surprise. Derek didn't mean to bring it up, but he was on his weekly call with Edwin (yeah, he keeps up with the kid-what of it?), and Casey was in the dining hall sitting alone. She had spotted him, and he, her.

She straightened a little as he had neared her table, and he thinks maybe she'd snap at him for even thinking of sitting with her (because all this talk about how Derek knows Casey; well, she knows him; it's not easy to pass up an opportunity to Casey-bait). Derek tore his gaze away and breezed right by. She's been staring daggers at him ever since.

Edwin happened to call, and the whole thing came pouring out.

"Yeah," Derek says, shoveling mashed potatoes in his mouth. No meatloaf this time; he had learned his lesson.

There's a long pause on the other line before Edwin asks, "Why?"

"You seem shocked."

"I am shocked. Baffled. Flabbergasted. Pick a synonym, and that's me right now."

Derek huffs, sipping his water and turning a bit so he can't see Casey out of the corner of his eye. "It's not a big deal." If Edwin thinks it's a big deal, he might take it one step further, and then... "I don't want to deal with her either, so it works out."

"Uh-huh." Derek notices the doubt in Edwin's tone. The distance has made Edwin brave. "It's just that getting on Casey's nerves is one of your favorite pastimes. I would have thought that her law-making would annoy you enough to want to break all those rules."

Edwin is right, of course, but there is more at work here. It has nothing to do with Casey's fierce insistence on wanting nothing to do with Derek. Something else, and he'd share what it was but, frankly, it's nobody's business. "I've grown," Derek says dryly.

"Sure," Edwin snorts. Derek makes a mental note to remind him of who the older brother is on his next visit home. "I'm surprised she thought it would work," Edwin continues, only willing to toe the line rather than cross it.

"So am I," Derek admits.

"So. If you're following Casey's rules, why is she mad?"

"That is the mystery." Derek sighs, stabbing his pot roast. "I don't pretend to understand the way her crazy brain works."

"Well, if you can't figure it out, I'm not sure anyone can."

"Maybe I broke a rule. Unintentionally, because I can't think of one that I might have broken, but..." He shrugs, even if Edwin can't see it. "It's the best explanation I've got."

"I can see what Lizzie says?"

Derek smiles, mentally crossing off the lesson-teaching from his to-do list. Edwin isn't so bad. Finished with his food, Derek stands to dump his plate on the conveyor. He catches Casey's eyes again. She narrows her brows. "Yeah. Let me know what you find out."

Lizzie isn't much help. According to Edwin, the only thing she can get from Casey is that she's confused as to why Derek is following the rules. "Lizzie points out that Casey should be glad you're doing it because now you can both move on from each other. Apparently, that resulted in a very long-winded rant that Lizzie didn't quite follow." Edwin takes a breath, "but apparently, Casey thinks that you're so well-behaved because you're planning something."

"I'm not."

"You're not?" Edwin confirms.

Frustrated, Derek rolls his eyes. "I'm not."

"I'll pass it along."

It stays like this for about a month. From Casey's mouth to Lizzie's ear, to Edwin's ear, to Derek's - Casey doesn't believe in Derek's innocence. They don't talk, though. There's a small exception where they do in class, and that's only because it's so tiny that he can't get away with always sitting silently. Even then, it's only about class materials. Sometimes, when she's particularly upset about a character in a reading, he thinks she might be directing it at him.

"You feel very strongly about this," Professor Adams says, sitting criss-cross on his desk.

"When someone behaves out of character," she says, shooting Derek a look, "it's natural for the audience to react negatively to it. It would be one thing if the character had a redemption arc or a gradual change but one day acting a certain way and the next, changing their entire being - how are we supposed to know what to expect?"

"I think you have a point," the blonde next to Derek nods. "I mean, there's the idea that we don't want a story to be predictable, but something is comforting in having some parts be a little predictable. The characters should act as they always have."

Casey's eyes are on Derek as she says, "Exactly."

Derek continues his pattern of good behavior, even going out of his way to avoid Casey. There's not a knife twisting in his stomach at the sight of her eyes telling him to follow the rules and Stay Away, so that's not the reason. It's just easier than trying to remember all the rules.

A month and a half into the first semester, Derek is surprised to find Casey on the other side of his dorm room door, her foot tapping angrily, her arms crossed. He tries to think about what rule he could possibly have broken (there are so many, he never did get around to reading the entire list), but nothing comes to mind. "Casey," he greets casually.

Casey tries to respond. It's almost comical the way her mouth opens and closes several times before she lets out a frustrated grunt.

"Right," he says, straightening. "As fun as..." he gestured at Casey in all her Caseyness (he didn't miss any of it) "... this is, I've got things to do. Whatever you're mad at me for will have to wait, although I can't imagine what, seeing as I haven't broken a single rule-"

"That's just it!" Casey explodes. She takes a deep breath, lowering her voice, and repeating, "That's just it."

"What?"

"You haven't broken a single rule."

Derek blinks at her, confused. Casey has done a lot of things that most people wouldn't understand, and he usually catches on to her crazy way of thinking. This time, however, he's lost. "I- what?"

"You've followed all of the rules! You've kept your distance. When you see me, you don't acknowledge me. You haven't shown up at my dorm, there have been zero pranks."

"So. You're mad that I'm... following the rules?" Saying it out loud didn't make the whole thing make any more sense.

"Yes!" She says, throwing her hands up. "Well, no."

"What?" Derek shakes his head. He doesn't have time for this. Turning, he heads back into his room, ignoring the fact that Casey has followed in after him.

"Are you planning something? Lulling me into a false sense of security?"

Laughing, Derek starts shoving his things into his backpack. "No."

She stops. "You're... you're not?"

"Nope."

"Then, what are you doing?"

He pauses, turning to look back at Casey. "I'm following your rules. I'm not sure what's so hard to figure out about that?"

"Why?" She insists, angry again.

"I don't get why you're so mad about this. These were your rules. This is what you wanted. I sat in that car, and you went on and on about not wanting me around, and I listened. You don't get to be pissed that I followed your stupid list of rules." It was a typical Derek and Casey standoff. They were inches apart, he was breathing heavily, and there was that spark in her eye. "I don't even get why you're pissed."

"Because you followed the rules." Casey shoots back.

Exasperated, Derek runs his fingers through his hair. "You're crazy." He shakes his head. "I mean, I always knew it, but this confirms it. You're nuts. How does that even make sense?"

"In what world does Derek Venturi follow the rules? I thought you'd break them after the third day, at the latest!" She shifts away from him, pacing his room. "Tell Derek Venturi not to do something, and he'll do the opposite. If there was one thing a person could count on, it was that Derek Venturi was not going to follow the rules." So maybe he wasn't imaging that conversation in English class being about him.

It's then that Derek finally understands.