Puck
He's had his suspicions for a while. Puck's a lot more observant than people would think, and he bides most of his time observing McKinley students. Sometimes he's just wondering which one of the Cheerios would be most likely to slide into bed with him that weekend. But sometimes, he's just trying to figure people out. He knows most people in Lima keep secrets. It's a small town, and nobody wants to fess up to being different. So he knows that there's people hiding things, people pretending to be something that they're not. Because high school is scary, and nobody wants to be an outcast.
Which is why it doesn't really come as a surprise when he figures out that Karofsky's kinda into dudes. It's in the locker room after a particularly grueling practice, and everyone's just lounging around in their underwear, trying to let the sweat dry before they get dressed. Puck's stark naked (he's proud of his body, damnit, and is totally cool just letting it all hang out) and leaning back against the cool metal lockers when Karofsky comes in. He sees the hockey players eyes scan down his body, and his pants twitch a bit. Puck raises an eyebrow, but Karofsky turns and heads for the showers without meeting his eyes. He doesn't say anything, of course. He knows he's hot and most of the guys here would fuck him in a heartbeat if they got the chance.
It's not till Hummel joins the football team that Puck realizes that Karofsky is legitimately gay (or bisexual, at the very least). Kurt realizes that the other guys are uncomfortable with his flamboyant self, so he changes in the corner quickly, not looking at anyone else and rushing out as soon as he zips up his skinny jeans. Naturally, he can't see Karofsky's gaze. But Puck does, and it freaks him out a little. As Kurt slides off his pants, Karofsky practically leers at him. He bends over a bit, and Puck glances down to see how other areas of Karofsky's body are reacting. He nearly drops his jaw when he realizes that the jock has a pretty serious erection. He stares for a second before it hits him, that he's looking at another dude's dick, and he looks away quickly. Kurt's dressed now, in some shiny silver button-down and jeans that have got to be cutting off circulation to his legs. He slams his locker shut, slides his bag over his shoulder, and saunters out. Karofsky's eyes follow him, and Puck totally sees the darkening of his face before the dude takes off for the showers.
Shit.
Puck never would've suspected that Dave Karofsky would be into Kurt Hummel. He kinda figured the dude, when he finally came out, would hook up with some athletic dude from a different school, and Hummel's pretty much a fairy. But then again, Kurt does wear some pretty form-fitting clothes, and Puck knows that it's always the girls in the tight skirts and sweaters that he checks out. And that's all it is, right? Karofsky's only staring at Hummel's ass because he wears tight jeans and shit.
But then he starts seeing stuff more often. Like when Karofsky and Azimo shove Hummel into lockers and throw him in dumpsters, Karofsky's hands linger a bit too long. He pushes up against his chest and grabs him around the hips. It's way creepy, but Puck doesn't really know if he should do something about it. And then he gets stuck in juvie and literally can't do anything about it, unless he wants his waffles stolen again.
When he gets out, though, he notices changes. The rest of the Glee Club isn't really getting bullied anymore. It's turned into the Kurt Hummel Bullying Show, Karofsky's the ringleader. Kurt gets shoved into lockers on an hourly basis, and sweet Jewish Jesus, that's gotta bruise like crazy. He gets taunted more often, and Karofsky's taken to standing way to close to him.
He knows something's really gone down when he sees Karofsky storm out of the locker room looking near tears one day. He stares after him confused for a few moments before the door swings open again. Kurt stumbles out, face pale and eyes wide. He heads the other direction, practically flying down the hall and through the front doors. Puck squints after him for a moment. Rehearsal started in five minutes, and Hummel had just taken off. He's pretty sure the kid has never missed a glee practice, like, ever, so the fact that he just left without a word was pretty bizarre.
So he looks curiously at Artie when he comes in and informs Mr. Schue that Kurt wasn't feeling well. He knows there's something else there, but Artie looks straight ahead without providing anymore information.
The next day, Puck's standing in front of his locker with his arm wrapped around Santana when he hears a slam and turns just in time to see Hummel crumple to the floor. Karosky's walking away from him, looking back with a smirk on his face. But when he turns to face forward the smirk fades, and he looks almost...sad? He doesn't really have time to think about it much, because Santana reaches up press her lips against his, and he can't resist the ladies.
He's half-asleep the night after the wedding. when he figures it out. He starts picturing that day outside the locker room. Karofsky, running out with red eyes and clenched fists. And Kurt, following shortly after looking terrified and holding a hand to his lips. His lips.
He sits up straight, and weird shit keeps coming together. Karofsky's increased vendetta against Kurt, Kurt's withdrawal from everybody, the damned locker room...
He doesn't even have time to do anything about it, because the next day Kurt waltzes into the choir room and announces in a shaky voice that he's leaving, Karofsky's coming back and so Hummel's transferring. Everyone protests, Finn looking pissed as hell and Mercedes looking like someone just killed her dog. Schuester looks depressed, and Rachel starts freaking out over broken harmonies or some shit. Kurt walks out without another word, and Puck has half a mind to storm after Karofsky with a baseball bat. But he's still on probation, damnit, and he really doesn't want to end up in jail for killing the douche.
So he pretty much develops his own personal vendetta against the dude, and makes sure to tackle him in practice whenever possible (even though that's totally not his job and he's pretty sure Beiste is starting to catch on). And he may have accidentally shouldered him into walls a few times, and his foot might have ended up getting in Karofsky's walking path on a few occasions, but the dude deserves it.
It's two weeks after Kurt leaves when he catches the bastard alone in the locker room. He marches up to him, arms crossed over his chest.
"What the hell do you want, Puckerman?"
Puck glares at him for a moment.
"I know what you did," he states. He's kinda hoping his vague statement gets Karofsky to admit to something, because the dude really isn't too bright. He's rewarded when Karofsky stiffens and meets his gaze, fear forming in his eyes.
"The fag told you?" his voice croaks on the slur.
"Maybe. Maybe not. Maybe I didn't know a thing, and you just implied that there's something going on. So tell me what the hell you did to him."
"I didn't do shit, man! Same stuff you used to pull, dumpsters and shit."
Puck growls and punches the locker in front of Karofsky's head.
"You know that's bull, man! I've never seen the kid so terrified before! Nothing I did ever affected him like that. What the fuck did you do?"
Karofsky scoffs at him.
"I don't need this," he says, grabbing his jacket and turning towards the door.
Puck waits until his hand is on the doorknob before speaking again.
"You kissed him, didn't you?"
Karofsky freezes. When he turns, he looks furious.
"Look, Puckerman. He kissed me, alright?"
"Really?" Puck raises an eyebrow, "Cause I'm pretty sure he wouldn't be into you."
Karofsky's face reddens.
"Let's face it, dude. He's kinda out of your league."
That does it for Karofsky. He storms back over to Puck and raises a fist, slamming it into the wall just to the left of his skull.
"I swear to God, Puckerman, if you tell anyone about this-"
"Stop messing with the Glee kids, got it? No more slushie facials, no more dumpster dives, no locker shoves. Don't even look at them funny anymore. And if you ever see Kurt again, you will apologize to him. He's a cool dude, he doesn't deserve your shit."
Karofsky backs up a bit.
"And I swear, Karofsky, if you do anything to get on my bad side ever again, I'll out you to the entire school, got it? The entire school."
Karofsky nods slightly, and Puck takes it as an excuse to leave.
"Puckerman? How's—how's he doing?"
"Not that it's any of your business," Puck says, looking over his shoulder, "but I'm pretty sure he's actually happy at Dalton. And I'm pretty sure he might have a boyfriend. Who's way cooler than you, by the way."
Karofsky looks devastated, and Puck takes off before the dude starts crying.
'Cause he just can't deal with tears, dude.