Kirishima might not be first in the class but he's way smarter than to take Bakugou at his word about exactly what he's ready for during his next heat.

Kirishima's sure he's the Prince Charming of this story. Bakugou is not exactly the princess he's been expecting. (spoilers through the 90s of the manga)

Obviously this was all the fault of Katsuki's perv classmate and his aerosolized heat trigger. Once Katsuki gets his life back under control, everyone will forget all about this moment of weakness. Nothing will change. He won't let it change. After all, he's going to be the number one hero one day. Just because omegas never go pro, that doesn't mean it isn't possible, right? ...right?

Kirishima tries to help Bakugou out during his first heat, and things get complicated.

On the way back to the mountains to get Kirishima some remedial dragon fire-breathing lessons, Bakugou agrees to let Kirishima scentmark him so the other dragons don't get any fresh ideas (if 'agrees to' means 'is completely into it').

“It’s just not manly to leave your bro like this, after knowing he can’t do anything about it, you know?” Kirishima blabbers some kind of bullshit excuse, and the worst part is that it looks like he’s totally convinced of it. “It’s just a— handjob,” he stumbles over the word, the weight of what he’s proposing hitting him all at once, but he doesn’t stop. “—but if you're not into it we can pretend I didn't just say that.” Bakugou has to restrain himself from saying that he’s very much into it. Like, a lot, really. a.k.a Kirishima learns the reason behind Bakugou's grumpiest days, and he offers to help because that's what best bros do.

Prompt from the Kink Meme: Kirishima gets oversensitive after using his quirk too much, and somebody (Bakugou) fucks him until he's so overstimulated that he cries

“I already told you, I’m not gonna break, you don’t have to go slow.” “What if I want to, though?” Kirishima murmurs quietly. “What if I wanna take my time with you?” Bakugou shivers.

It's involuntary the way his knees seem to buckle and give out and throw him onto the ground. It takes him a moment to realize he's hyperventilating, tears running frustratingly down his cheeks as he rubs his clothed erection slowly against the carpet. / Or the one where Bakugou Katsuki is presenting as a fucking omega, and there's nothing he can do to stop it

Apparently bad things really did come in threes, Bakugou thought wryly, as the firefighter closed the distance between them. Tall, tan, and jacked as hell, he was a strong contender for the single hottest person Bakugou had ever seen in his life. He was covered in soot and sweat that contoured his ridiculous muscles, making them look even more defined, and his clinging black tank top didn’t do much to hide the rest of him, either. His strong cheekbones and jaw had smears of black on them too, and Bakugou had the absurd thought that he wanted to lick them clean. Bakugou didn’t think it was possible, but he felt himself turning even redder. He would barely have been able to talk to someone as attractive as him on a good day, and he was about to have to explain to him that he’d tried to burn down his fucking house because he was being petty on Twitter. Super.

Kirishima lived for these moments. Bakugou pressed back against his chest, thighs spread over Kirishima's own, hands sparking and popping over Kirishima's shoulder while the redhead worshipped Bakugou's body with his hands. ----- (Alternately summarized as: another fic where Bakugou can't jerk off but his bro helps and then feelings show up)

When people start dying on the ship, Bakugou has to rush to find out what is going on. Along the way, he has to deal with the grief of losing the people he loves, and the fear that he might be next. At least, he has Kirishima by his side...

“Bakugou?” Bakugou shuffled on his feet, hovering over Kirishima and looking at the ground with stormy eyes. He glanced up to glare at Kirishima, a silent dare to call him out on his odd behavior no doubt. Kirishima forced himself not to tense. Whatever Bakugou wanted, he was about to show him and Kirishima had to get this right. Bakugou was all about showing and not telling. Kirishima nearly bit his tongue to keep in a squawk of surprise when Bakugou suddenly dropped to his knees next to him, shuffling forward until he could press his forehead to his thigh and hide his face against Kirishima's leg. Kirishima opened his mouth, questions on the tip of his tongue, and he barely managed to catch them before they could be given voice. Bakugou was trembling minutely, his entire frame so tense his muscles were twitching under Kirishima's gaze. “Just. Don't say anything,” Bakugou muttered, hands clenching in his lap tightly. “Please,” he whispered, a short choked sound.

Bakugou sleeping in the common areas like it’s no big deal seems to give everyone else permission to be just as bizarre, and little by little Kirishima starts learning things about his classmates he never knew.