BNHA Venom!AU (set in the US for ease of writing) where Izuku is a baby symbiote who accidentally ended up on earth. He ended up lucking into a host, an 8 year old Katsuki who’d been backpacking through the woods nearby where he landed.
Katsuki was always a bit of a weird kid. He didn’t get along well with any of the kids he knew, and his parents were rather distant, being high-end fashion designers meant they were always off across the world doing other things. As a result, when this weird alien attaches itself to him, he’s annoyed for sure, but he doesn’t panic, or call the police, or really do any of the normal things that a child should do in that situation.
He just kinda,,, puts up with Izuku. Sure, he’s pissed off and he tells Izuku to go away for awhile, but soon enough he learns to accept this weird alien that’s a part of his life.
Quickly, both he and Izuku discover that Izuku can help him do some pretty cool things, and Izuku needs lots of chocolate to stay healthy. Both things are just fine by Katsuki, and he ends up deciding that Izuku is actually alright in his books.
Some government agents snoop around, at least somewhat aware of an alien being in the area, and Katsuki quickly learns to hide Izuku from anyone else. He doesn’t want them to take his new friend and hurt him like they described. Katsuki manages to escape suspicion as not even his own parents know about his backpacking/hiking hobbies, and nobody really thinks a spoiled 8 year old kid of two fashion designers would be 10 miles out in the woods.
After that, life keeps going for Izuku and Katsuki. They learn to live together, how to adapt to all the various quirks of sharing a body, how to use the powers Izuku can give Katsuki, and how to keep Izuku well hidden.
It’s all going great, right up until Katsuki makes it into an extremely prestigious boarding school; UA. It’s his dream high school, but it’s so much harder to hide the fact he’s got an alien living in his body when he’s living with other teenagers, and the security at the school is much higher than what he’s used to. For fuck’s sake, the vice president’s kid goes to this school! There’s no way they aren’t getting caught.
Still, Izuku doesn’t want Katsuki to give up on his dream and encourages him, so Katsuki reluctantly goes to UA anyways, trying his best to keep Izuku hidden.
But there are... incidents. Not ones where anyone dies, but people seem intent on harassing Katsuki, from fellow classmates to muggers outside the school, and Izuku is very protective by nature. After a few of these ‘incidents’, a certain underground branch of the government starts getting suspicious again, launching an investigation into the school.
Meanwhile, Aizawa, Katsuki’s primary teacher, has started noticing lots of weird things about Katsuki. Like how he always has two columns in his notebooks, with one side full of carefully written notes and the other side filled with random excited musings in rushed handwriting. Or how sometimes he'll seem to become a different person for a moment, or how he seems to vanish at odd moments.
Weirdest of all is his homework/essays, where Aizawa would swear half the time it's a different person writing them (he only knows for a fact it's Katsuki doing both, since he's watched the kid write most of his essays at various stages in the library). The core ideals themselves seem to change between essays, and that’s what bothers him the most since he’s never seen that in a student before.
Being the good concerned teacher he is, Aizawa considered all the evidence before he called Katsuki into his office. Katsuki and Izuku are sure they've been caught.
Then Aizawa asks Katsuki if he's ever been tested to having multiple personalities.
And listen, Katsuki does not believe in faking mental disorders. But by this point, he's really worried that the government is gonna get Izuku, so when he sees the chance to excuse all his weird behaviors he latches on with everything he’s got.
So Katsuki explained, while trying to lie as little as possible, that he's never formally been tested (and he doesn't want to be), but there is another person living in his body named Izuku. Izuku is very shy though, and would prefer to not be mentioned/brought attention to. As such he asked Aizawa to not tell anyone else about him. Aizawa agreed easily, after all he doesn’t see any real risk from Izuku, and he’s happy that Katsuki was honest with him about the situation. He tells both of them that they can come talk to him any time if they need help, Katsuki agreed to that, and for a little while Izuku & Katsuki were safe.
I dunno exactly how the class finds out about Izuku, but I do know that Iida is one of the first ones during a tense situation. As a result, Katsuki and Izuku get to watch as straight laced goody two shoes Iida looks a government agent right in the eyes and lies his ass off to him to protect Izuku.
For this AU, the ‘Kidnapping’ happens when the government managed to catch onto Katsuki. They’re sure they’ve finally found the alien, and they move in to capture Katsuki.
Only, Katsuki gets wind of this ahead of time, so in a quick moment of desperation, he forces a weakened Izuku to transfer off of him and onto Shouto, since Enji is like the vice president in this AU and that makes Shouto much, much harder for a government agency to fuck with. Shouto is somewhat aware of what Izuku is, and he agrees to take the other to safety.
Poor Izuku can only watch as Katsuki is captured, while Shouto forces him to stay hidden for all their safety.
It's okay though, because the four people who know about Izuku and what happened (Shouto, Kirishima, Iida, and Momo) decide to break into the government base to get Katsuki back, with Izuku's help obviously.
They get most of the way through their super secret infiltration mission, but something goes wrong and they’re almost caught.
Until while they're hiding and the people search for them are a hair away from discovering them, an absolutely furious Aizawa just fucking waltzes in with the fucking president (All-Might for this AU) to demand his student back.
Now at this point, the agency has no actual proof of alien life. They have half connected incidents and eyewitness reports, but nothing definitive. When they took Katsuki into custody, they’d tested him immediately, sure that they were going to get their proof that way, but since Katsuki no longer has Izuku, the tests all came back negative. As such they have no real justification for having basically kidnapped him.
The agent guy or whoever has been chasing them tries to justify themselves by offering proof of 'Izuku' existence, specifically recordings of Katsuki talking to himself and referring to an ‘Izuku’ as well as written notes. Aizawa's responds that he was already aware of Izuku, that Katsuki had already discussed the other with him, and that he cannot believe they've detained a 16 year old on account of him having multiple personalities. All-Might backs Aizawa up, and shuts down the entire thing as there’s no evidence they’ve done anything but harass Katsuki.
That leaves the agents shit out of luck, so they have to let Katsuki go. The other kids sneak out the way they came in, before quietly joining Aizawa and Katsuki. Aizawa waits until everyone is outside and out of range of any cameras, and then he just turns on the entire group.
"Okay, so first of all, I'm extremely disappointed that you four decided to break into a government agency with armed guards before so much as texting me."
"We could've thought that through a little better, I'll admit."
"Katsuki was in trouble, but yeah, I guess we could've asked for help."
"I stand by what I did and I'd do it again."
Aizawa, actively chooses to ignore Shouto's comment for right now, but does still glare at him for it.
"Second of all, that man may be a fool, but I am not. Where's the alien?”
Everyone's quiet for a tense moment before Izuku makes his choice and pokes his head out from Shouto, introducing himself politely.
After getting a full explanation on what happened, Aizawa gives Katsuki a Look (tm) to which Katsuki defends himself.
“Listen, I did not lie, I just purposefully omitted that the person living in my body is also an actual alien from outer space.”
And Aizawa can’t really argue with that, so he just accepts that his class has an extra student in it from now on.
They let the rest of class 1-A in on the Izuku secret, since a lot of them had been coming close to figuring it out anyways, and so the entire class slowly adjusts to having an alien as one of them. It’s surprisingly easy to come to terms with, and as Izuku slowly starts coming out of his shell (ie hiding inside Katsuki whenever someone tries to talk to him), the entire class discovers he’s a total sweetheart & adore him.
