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Izuku knows he isn’t fully human when he starts growing sharp teeth. They make his mouth hurt and he feels the urge to bite into things for some relief.
He’s five years old and he’s confused, because he looks at everyone else’s smile and it doesn’t look like his. More importantly, Kacchan’s teeth don't change.
Izuku is the only one. The only one that's not like everyone else.
When he asks his mom she gets all quiet at first, then picks him up and puts him on her lap. She cuddles him close. Izuku feels like he's getting too old to cuddle his mom—he’s not a baby—but he still loves it, being held tight.
Then she opens her mouth to show him her own sharp teeth. Izuku’s eyes widen and he has a hundred questions but she asks him to listen first and then ask.
She tells him about her mother’s family, and how a long, long time ago they were given the gift to transform into powerful, monstrous creatures to protect themselves. Izuku gets hopeful that this means he has the power too—and he can use it to be a superhero like in the comics, but she says no. Over the years and years that passed, as her family married into other human families—that power was lost.
They can’t do cool stuff anymore. Kacchan would probably say “that blows” and he would be right.
Now all they have are these little quirks. Sharp teeth. The urge to bite. And eyes that glow in the dark.
“You can’t tell anyone that we’re descended from monsters, okay Izuku?” She makes him promise. “It’s not safe.”
Izuku interprets this as you can't tell anyone except Kacchan, and he nods. He understands.
She gives him a file that he can use to shave down his teeth from time to time, but before he uses it he goes to the playground the next day to tell Kacchan the truth about what he is.
He can’t keep secrets from him. It feels wrong to keep anything from his favorite, closest person. They haven’t got along as well lately, but Kacchan is still his best friend.
“I gotta show you something,” Izuku says and he opens his mouth wide and points to his teeth. “I’m…I’m a monster.”
He waits for Kacchan to call him a Deku again or something, but he just surges forward and grabs Izuku’s face to look closer.
He's frowning, making all sorts of faces while he inspects Izuku’s mouth. “Huh? No fair! How’d you do that?”
“I didn’t do anything, Kacchan! That’s just how I am.”
Kacchan stands really close. His cheek looks soft and pink with the cold. Izuku wants to bite it so bad. With Kacchan's fingers in his mouth, holding it open to get a better look at his teeth, he can barely hold it back anymore.
He wants to bite. Wants to bite his Kacchan.
“Sorry, please excuse this!” he manages before wrenching himself out of Kacchan’s grip and taking hold of his face, then sinking his sharp teeth into Kacchan’s cheek, hard enough to leave indents but not enough to break the skin.
Kacchan cries out and he shoves Izuku off of him in shock, then stares.
The bite satisfies the urge but Izuku feels awful. He can't believe he just did that. He takes a step back, wanting to run, but he can't. He wants to cry.
He bit Kacchan in the face.
Now Kacchan will never want to be his friend again.
Kacchan opens his mouth and Izuku’s sure he's going to say something mean, and this time he feels like he deserves it, but as soon as he’s lowered his face in shame Kacchan is tilting it up again and now he’s biting Izuku right back.
It hurts. Kacchan’s teeth aren’t that sharp but he bites hard. Izuku comes back to his senses after a second of shock then pushes Kacchan backward. “Ow, Kacchan!”
“You think you can just bite me and I won’t bite you back? I don’t care what you are, that doesn't matter, anything you do I can do too. And I will.”
Izuku raises a hand to feel the spot where Kacchan’s teeth grazed his skin.
Wow.
Kacchan doesn't care if he’s a monster or not. This doesn't change anything between them.
“You can’t tell anyone though,” Izuku makes him promise. “You can’t tell anyone about me. Okay?”
They’re both staring at each other, slightly surprised. What Kacchan has to be surprised about, Izuku doesn't even know. His bite was deliberate.
Kacchan is really weird sometimes.
“Fine. No one’s gonna wanna know your stupid secret anyway.”
He sounds like he doesn't care, but he’s the one all through middle school who’s always telling Izuku his “fucking fangs” are showing when he goes too long between filing them down.
He’s the one who shoves Izuku's beanie down over his eyes when it gets dark and his eyes glow. He’s always got a spare pair of sunglasses.
At sixteen, he lets Izuku softly bite into his exposed shoulder in the school locker room when there's no one around. He keeps biting him back, even when there's no urge, no monster blood in him that should make him want to. He bites Izuku even more than Izuku bites him, for some reason. His teeth graze against Izuku's wet skin after a shower, and when they're alone sometimes Kacchan will lift up his scarred hands and nibble and kiss his wrists.
Izuku understands what this is. Kacchan knows what he needs, Kacchan always knows what he needs, and Kacchan is taking care of him by biting back.
By holding Izuku’s head against his chest while he chews on Kacchan’s collarbone and sounding smug. “That’s good. That’s better, isn’t it?”
They do this before even kissing on the mouth. Kacchan goes with him toe-to-toe, bite-for-bite and it’s this ritual of theirs that makes Izuku feel like him being a monster kind of isn’t that weird. Or even if it is, it’s safe.
Kacchan keeps the secret for years. Izuku’s sure he’s never told anybody. He almost seems to like it, that Izuku’s need for him makes it so easy for him to go on and reciprocate. But Izuku pushes that thought to the back of his mind, thinking that Kacchan might like him back is sort of like staring directly into the sun. It would be really great for a second but burn him in the end. It’s dangerous.
When they’re seventeen, Izuku starts kissing the places he’s bitten on Kacchan’s skin—out of love, not an apology that Kacchan never asked for—and never to be outdone, Kacchan grabs him and kisses him hard on the mouth, then soft, then everywhere.
When he watches how Izuku's green eyes shine in the dark, with a luminescence all its own, he calls him a “dumb glowstick” a “living goddamn lava lamp” and Izuku takes each nickname to heart as affection. And holds it close to his chest. Since Kacchan keeps offering his own glycerin-soft skin up for biting, and he's always passed along love as rudeness.
“You better not go to anyone but me when you've got the urge to gnaw on something,” Kacchan says once, voice rough with sleep and one arm over Izuku's waist.
They’re eighteen years old and he’s Kacchan's “fucking diluted vampire” a “monster lite.”
They live together now, which means Kacchan is the one who helps him shave down and check his teeth instead of a mirror.
They’re facing each other in Kacchan's bed. No shirts. Izuku had his teeth on Kacchan's collar bone before, and he's got Kacchan's teeth marks on his own neck and chest. They were all over each other before they got drowsy, and now Kacchan is being soft like he always tends to be when he's exhausted.
Saying he doesn't want someone else taking care of Izuku's needs like that. He needs to be the only one. He needs to hear that he is.
“It is only you,” Izuku says, and he thinks back. “I think that's been true since the first time.”
“You dork. You didn't tell me all of it then,” Kacchan complains. “You showed me the teeth but you didn't tell me your eyes look like green fucking fireflies.”
Fireflies. Usually he gets “glow-in-the-dark goo” or “radioactive waste.” This is openly romantic. Is-is Kacchan okay?
They used to hunt fireflies together. Kacchan had this net. Izuku cherishes that memory and he wonders if Kacchan looks at his eyes and thinks about it too, if fireflies is the comparison he makes.
“I thought you’d think the teeth thing was cool, Kacchan. Kind of like I was a vampire. The eye thing is just–”
“Fucking cute,” Kacchan finishes.
And Izuku thinks his chances of surviving the rest of this conversation are maybe one in five.
“Huh?”
“Shut up. It’s real fuckin’ adorable,” Kacchan continues. “And I don't think the teeth thing is cool because they’re sharp. Kirishima’s got kinda sharp teeth too, hell, so do I. That’s a whole different thing. I liked it when you just lost it and bit me.”
“Kacchan, I was five.”
“I know, so was I. I’d never felt something like that before and when I saw your face after you did it I just had to do it back. I feel like…sometimes you're so fucking cute I don't know what else to do. And since you do it too…” Kacchan trails off.
Oh my god.
All this time. “Kacchan, do you have biting aggression towards me?”
“Fuck. Maybe. I don’t fucking know. I used to think I felt so mad because you got on my nerves or you were annoying but once I got back to that day on the playground I think it was the only time things felt right with you. With me. It’s not…it’s not even sexual. Or, it wasn’t then.”
Kacchan is blushing now. Which always makes Izuku want to bite him even more. He likes the flush of pink, almost red at the ears. He likes how warm Kacchan’s face gets.
“I think it’s just something we do, you and me,” Izuku says. “And I'm fine with that.”
There's nothing wrong with a few love bites.
Izuku still knows he isn’t fully human. But it’s never mattered. He and Kacchan are together, and they don't need to be like everyone else.