The day that Chuck catches them together, Chuck and Blair have already been together for two years, and the moment isn't anything to speak of. If Chuck didn't know them both so well, he wouldn't have thought anything of how close they were standing, or how Blair watched Serena's mouth when she talked.

If he hadn't noticed that, he wouldn't still have been standing in the doorway of Blair's dorm room when Serena laid her head on Blair's shoulder with a tiny giggle, stopping very briefly to kiss her lips first.

Chuck mastered teenage experimentation when he was still barely a teenager. He's mostly shaken off the need to know what everything was like, that feeling of being shamelessly curious about everything under the sun. But he knows what it looks like.

Serena van der Woodsen strokes his girlfriend's hair, and Chuck can see this isn't that.

He knocks on the wedged-open door, and they both jump at the sound.

---

Chuck's very first thought is that he isn't going to lose Blair to Serena van der Woodsen. His second thought is that maybe she's already gone.

Chuck thinks Blair is probably the love of his life, and that scares him more now than it did when they first started dating.

Chuck's father only ever loved one woman. Living without her pretty much broke him, and he never loved anyone after that. Not really. It's a shitty example, Chuck knows, and still it's the only one that he's ever had.

---

When Nate fell in love with Serena, both times that he fell in love with her, Chuck knew it was going to end badly. You couldn't ever talk Nate out of following his heart, though, so Chuck didn't try.

---

"Maybe it's nothing," Nate says. "Like you and me."

Chuck kissed Nate when they were both thirteen. They hid together in a closet at the Archibald townhouse, trying to keep down stoned giggles, and when Nate finally stopped laughing, Chuck kissed him. It was a few short minutes of messy, teenage making out, and they never did it again.

Chuck would bet his hotel, his entire future fortune, that Blair and Serena's story takes much longer to tell.

Nate wasn't the only guy Chuck ever kissed, but he was the only friend, until four years later when Blair Waldorf stepped on stage at Victrola. He talks to Nate about it because Nate loves them all, even though they've all broken his heart at least once. He tells Nate exactly what he's thinking of doing.

"Dude." They're both completely drunk. It's the only way that they ever really talk about anything important. "That's a terrible idea."

Maybe, Chuck thinks, that's part of the appeal.

---

He spends weeks planning ways to knock Serena out of the picture. Serena notices him staring at her at Lily's dinner table, frowns, and then smiles. She turns to Humphrey, who's telling another unfunny joke, and runs her fingers through her hair.

He makes plans. The simple, the complicated, the foolproof and the downright cruel. Serena smiles at him at dinner, and Chuck never puts any of them into motion.

---

When Chuck kissed Serena, they were fourteen and both wasted. They weren't exactly friends then, but they aren't exactly friends now, either. It was ninth grade, and they were just supporting players in the Nate and Blair show.

At the time, it had seemed like Nate and Blair would last forever.

It was a party, thrown by the senior lacrosse captain, and the townhouse was crowded with wasted St Jude's seniors. They were the only freshmen present. The senior boys loved Serena, and Chuck went wherever the fuck he pleased.

When he ran into Serena in the kitchen, she'd just come in from the rain. Her gold-colored dress was soaking wet and stuck to her skin. She shook the water out of her hair, and didn't seem to care.

"Got any booze?" she asked him, and Chuck handed her the bottle of vodka he'd swiped earlier in the evening from the Palace bar.

"Awesome," she murmured gratefully, and then climbed abruptly into his arms, dripping rainwater over his tailored shirt and pants. "This party is boring."

It had been, Chuck agreed. But it wasn't any more.

---

On Monday in the school courtyard, he thought she was ignoring him. On Tuesday, he realized that she honestly didn't remember, and on Thursday she caught him staring at her and made a face. "What is your problem?"

Chuck didn't think about it after that.

---

Chuck knows that he's not a perfect boyfriend. But he does try to make sure that Blair has everything. He asks her the question over dinner in his suite, steak and fries and '95 Dom Perignon.

"Do you remember our game from the summer?" The first summer, Manhattan-hot and etched into memory.

Blair nods, frowning. They haven't played that particular game in a long time.

They play other games instead. He likes her in outfits, anything he can take his time getting her out of. Blair likes to have him at her mercy. Handcuffs, blindfolds, leashes. Anything that will keep him in place, until he's tethered to the bed and pleading.

"Are you bored?" she asks him. "I'm sure we can think of some way to make things more interesting."

Chuck shakes his head. "The question is, are you bored?"

"You want me to play the cheating cad?"

"Why not?" Chuck reaches across the couch, and grabs her wrist, pulling her into his lap. "I could be the spurned, jealous lover defending his honor."

Blair seems to like that. She smiles. "Would you fight for me?"

"I would fight anyone for you," Chuck tells her, honestly. "Anyone." His arms wrap around her then, pushing her down, pinning her to the couch. "With my last breath."

Blair gasps underneath him. This is the part that's always been easiest.

---

Two nights before Blair's twenty-first birthday, Chuck and Serena hot-box the bathroom in her suite at the Empire. Serena strips down to men's boxers and a tank top, and sits on the edge of the bathtub while she smokes with him.

Chuck sits on the bathroom counter. It's kind of nice living with her again, but he'd never say that out loud. He keeps threatening to kick her out, but never does. She keeps threatening to leave, but never does that either.

"Blair sent me her wishlist," Serena says. "Again."

"I already picked something out for her."

Serena wrinkles her nose. "Bad idea, Chuck. You know Blair likes to pick out her own gifts."

"I know she does," is all Chuck says.

Later, he kisses Serena good night, and she's stoned and she giggles against his mouth. "Chuck. Boundaries."

Chuck's only answer is to put his hand on her waist, and invite her to breakfast.

---

The next night, he takes Blair to Nobu and orders her favorite sake, and then he closes down the club, just for her. Birthday parties still make Chuck irrationally uncomfortable, like he's turning eleven again and his dad has forgotten. Again. But birthday parties are Blair's favorite thing, so Chuck throws her one that's even more expensive than the year before, and kisses her before she gets out of the limo. "Happy birthday," he says, and she smiles at him.

The bartender serves a plate of 21 tequila shots alongside the cocktails. Dan, Vanessa, Serena and Nate all help, but Blair is swaying in her 4-inch Louboutins by the end of the night. Her eyes are bright, and she and Serena spend half the night on the dance floor like they're thirteen again, dresses swaying and hands touching.

---

Chuck tells Serena he needs help getting Blair upstairs. Serena lost her shoes, earlier in the night, and now it's not clear who's helping who. They giggle too loudly in the elevator and Chuck watches them. He's feeling like they look, warm and slightly stupid.

Maybe this will be easier, drunk. In the morning, if it's a mistake, they can blame it on the Patrón.

Chuck closes the door of his suite, and pours them all another drink.

---

"You like my girlfriend." He doesn't waste any time before saying it.

Serena looks up like she's been hit. She presses her lips together, shaking her head. "Blair's my best friend."

"Please, van der Woodsen." Chuck is holding Blair's hand. For once, she doesn't seem to have anything to say. "You guys were never friends. You like her. You've always liked her."

There's silence, in which Chuck drains his glass. Then he reaches for Blair's and takes it out of her shaky grip, before she drops it.

"She likes you, too."

"Charles," Blair says, desperately. Chuck realizes for the first time that she's not pretending. She's really this scared, and he tightens his grasp on her hand. Sometimes the only thing to do was pretend that you believed in happy endings. Blair's the one who taught him that, or maybe they taught each other.

Chuck looks at Serena, and says more firmly, "You like my girlfriend."

"Whatever," Serena says, a shrug that's kind of an admission. She's looking down at the carpet, looking nervous and lost. Chuck remembers that Serena doesn't believe in happy endings, either. "I guess."

"That's hot," Chuck says, and they both roll their eyes at him.

Chuck adds, "I don't have to stay."

"She's your girlfriend," Serena points out.

"Yeah, she is," Chuck says, because none of them get to forget that, not tonight and not ever.

---

When Serena leans forward and kisses Blair, Blair shivers. Chuck knows that feeling, the wanting.

"Stay," is the only word Blair manages when they pull apart. To Serena, to Chuck, to both of them standing here together.

"If that's what you want," Chuck says.

Serena glances at Blair, hesitating, and Blair nods slightly before Serena kisses him. It's warm, tequila-soaked, real and full. Blair never lets go of Chuck's hand.

---

He'd fight for her with his last breath, but in the end he doesn't have to fight at all.