Though it doesn't seem like it, Bakugou is actually a very reserved and isolated person. He doesn't bother to get close with any of his classmates, doesn't chime in with his opinions during their class discussions, tries to be as removed from everyone else as much as he possibly can. In fact, the only times he ever even talks to his classmates are when he yells at them to shut up because they're being too annoying and loud.
Which is why it infuriated him beyond belief when Kirishima just up and decided one day that he was his friend now and Bakugou just had to deal with it. Where he once would sit alone at lunch Kirishima now sits with him, when he would ignore the conversions people were having during break Kirishima included him and asked his opinion, at the end of the day walking home where he once would have just his thoughts as company he now had Kirishima talking his ear off about this or that. Everywhere he went it was Kirishima, Kirishima, Kirishima.
His carefully constructed isolation chamber formed a door and barrelling through came Kirishima, ready with a cheesy joke or story from his middle school days. And it pissed him off beyond belief. Because now other extras were starting to jump on the train of pissing Bakugou off, following him around like lost puppies. He now had to deal with a gaggle of idiots everywhere he went during school and it was all Kirishima's fault.
But somehow, somewhere along the line, Bakugou started actually enjoying his time with the shitty haired loser. No longer was Bakugou forced to stay in his seat during lunch when Kirishima came over, or needed much more than a little prodding to give his opinion on something and join in on conversations, or stayed silent on the walks home. For that Bakugou could give respect, for being able to do something soother person has ever been able to do since he was four. Become Bakugou's friend.
Of course, Bakugou would rather give up his chance at number one than ever admit that, but Kirishima had truly managed to worm his way into Bakugou's cold heart, filling it with warmth and happiness which left him fidgety and uncomfortable. Kirishima didn't do anything grand, didn't prove his worth by beating him in a fight or show he was as capable as Bakugou, he just inserted himself slowly into his life until he was just suddenly there, was suddenly regarded as a friend, an equal.
Maybe that was why he chose to grab his hand that night, maybe that's why he let loose a chain of explosions that left his arms aching for days just to hold his hand and escape into the night. Maybe he didn't grab Kirishima's hand that night because it was the best option of escape, maybe he did it because he trusted him, trusted him to not be rescuing him because he saw Bakugou as someone in need of saving, but because he was an equal helping another equal out.
Maybe that's why he was sitting on Kirishima's bed, pretending to read a comic book but instead staring at his friend and how the light shined through his bright red locks, turning it from a cherry red into a softer, blush-like color. Kirishima glanced up from his own comic, giving him a small smile, one sharp tooth resting on his bottom lip. Bakugou felt warmth bloom in his cheeks and ears, snapping his gaze back to his comic and forcing himself to continue reading.
A few minutes pass without the other saying a word, until Kirishima lets out a heavy sigh, flopping down onto the ground. Bakugou looks away from his comic and down at him, his narrowed eyes telling him, what the fuck do you want shitty hair. Kirishima thinks Bakugou would still call him shitty hair even in his brain. It just seems like a Bakugou thing to do.
Kirishima finally relents after a minute or two of staring. "Ah man, I don't know, I just feel really bored for some reason." He shifts his stare to the ceiling, looking at all the cracks.
Bakugou sneers, "What, is my presence not enough excitement for you?" Despite the scowl on his face, Kirishima can tell he's just joking with him, in his own Bakugou way. Kirishima lets out a huff, a smile coming across his face.
"Nah man, I'm pretty sure you could keep a group of elephants entertained for years. My mind just feels foggy, and I'm pretty sure all of my limbs are one second away from falling asleep. Sucks we can't go to the gym after seven, seems way too early for me." He rolls over on his side, letting his head rest in the palm of his hand, his elbow on the floor as he looks as Bakugou.
Bakugou scoffs and throws his comic down, crossing his arms. "Well what the fuck do you want me to do about it?" Kirishima bites his lip, seemingly thinking something over. Bakugou's gaze falls to his lips and the sharp teeth lightly biting it, entranced by the small movements his teeth make across the pink skin. He jolts as Kirishima suddenly jumps up and plops himself down on the bed next to Bakugou. "What the hell are you doing shitty hair?" He questions, tense from the sudden movements the other boy made.
Kirishima smiles slightly, though something seems off about it. He's nervous about something, which makes Bakugou even more tense and he narrows his eyes, suspicion clear in them. Kirishima ignores the look and shifts closer to Bakugou, until their noses are a hair width apart. He holds his breath, not daring to make a single sound. "Well," Kirishima replies in a low tone, breath ghosting over Bakugou's lips, "we could work out in a different way."
Bakugou lets out a shaky breath, staring into his half-lidded gaze, surprised by the boldness Kirishima just displayed. He lets a few seconds pass in impatient anticipation before surging forward and capturing Kirishima's lips with his own, falling forward and letting Kirishima's back hit the bed. He plants his hands on either side of Kirishima's head and sits himself on top of his thighs, all the while moving his lips against Kirishima's.
So, yes, maybe somewhere along the line of them knowing each other did Bakugou let Kirishima worm his way into his heart and call him an equal, a friend. Maybe during that friendship Bakugou had developed a feeling much stronger than friendship towards his shark-toothed friend. Maybe this friendship and the subsequent romantic relationship that followed brought him out of his isolation chamber and made him less reserved and alone.
But Bakugou swears up and down he's still a reserved and isolated person at heart, that it only seems like he's not because of Kirishima, and you better fucking believe him or he'll blast your chest open and use your ribs as a canoe.