Have thought it over further, Iâve decided on how this would work, at least roughly:
We push the ages ever so slightly for Aizawa and Shinsou. Nemoto has no canonical age, so he can be like 2 years old than Aizawa.
Mic dragged Aizawa to a house party when they were teenagers, Nemoto happened to be there too.
Nemoto is trans, but he had only just realized it at the time, and hadnât decided on making a physical transition yet.
Aizawa and Nemoto both got very drunk and ended up sleeping together. So drunk that the morning after, Aizawa barely even remembered it happened. They didnât even wake up together.
Nemoto never even got Aizawaâs name, let alone learned much about him.
Cut to a few months later, Nemoto finds out heâs pregnant. He has to debate it for awhile, but canât bring himself to terminate the baby. Still, heâs aware that heâs in absolutely no place to be a parent. He doesnât want to be one either.
He gives up Shinsou right after heâs born. He does fill out the paperwork, and he leaves his own name so if health issues or something come up, Shinsou could find him in the future. He gives Shinsou a brand new last name, with a note left behind telling him to make them his own.
Nemoto never tries to contact Aizawa about any of this. He has no chance of finding him again, and since he doesnât plan on keeping the kid, it shouldnât matter.
Shinsou grows up between orphanages and foster care. It gets a bit harder after his quirk develops, but only gets rough after Nemoto becomes a proper villain.
Police come to talk to him, very suspect, asking who heâs had contact with and even watching him for periods before they finally determine Nemoto really did leave him behind and has no interest in him. Shinsouâs file gets big red warnings stamped on it and his options dwindle as a result.
Even his teachers at school are given a warning about it, and while theyâre supposed to keep it silent, few of them do. Shinsou becomes known as a villain kid, no matter how hard he tries. Nobody in any of his classes trust him or want to be near him.
UA is the first school he attends where confidential shit is kept actually confidential. Itâs a drastic change for him. The school is well aware that he has nothing to do with his birth parent, and they donât let it change how they treat him.
Shinsou asks Aizawa on their third day of training together if he knows about his villain heritage. Aizawa confirms that he does, but says it doesnât mean anything. Shinsou is not guilty for having been born.
After Nemoto is arrested, Shinsou finds he can breath easier. The warning on his file is lightened, and that means he might be able to switch out of his current foster home. Itâs gotten easier since the dorms came around, but his situation still sucks.
Around this same time, Aizawa has gotten licensed as a sort of foster parent so he could help take care of Eri. Since heâs got the license, and heâs talked it over with Mic, he decides to see about taking Shinsou on himself.
Before he talks to Shinsou about it though, he wants to make sure itâs possible. The paperwork is fucking complicated, particularly considering that Aizawa and Mic are both pro-heroes which is a high risk career.
During the middle of all this paperwork, Aizawa gets hit by a radiation quirk and as a result he needs a bone marrow transplant and he needs one pretty fast.
His entire class, Shinsou include, jump to get themselves tested for donation (I know the limit is 18 but Iâm pretending itâs 16 for Plot). Most of them wonât work, but Shinsou ends up being an amazing match.
Aizawa reluctantly accepts Shinsou as his donor after heâs assured by several doctors that itâs a safe procedure for Shinsou and wonât put him at risk.
The entire donation process, the two of them keep getting mistake for a parent and child by the doctors. Itâs annoying to correct people, and neither of them is exactly sure why until one nurse bothers to explain.
Turns out Aizawa & Shinsou arenât just a good match, but their quirks are also heavily related. Itâs good because thatâs ideal for donors, but also strange since the two of them are insisting theyâre unrelated. Quirk types matching up outside of family members is extremely rare.
After Aizawa recovers, he and Shinsou are both somewhat curious about that. Shinsou admit he doesnât know who his other parent was, Nemoto wrote that he had no memory of them, so itâs possible that the two of them are somehow related through an unknown cousin or something.
They decided to take a DNA test to check, it takes a few days for the results to come in.
Aizawa opens up the letter for the test he did more out of curiosity than anything else and sees the words âBiological fatherâ staring back at him.
He goes through a lot of emotions in that moment. Confusion. Guilt. Feeling lied to. Disbelief.
And then he remembers literally the only time he hooked up in his teenage years. A time point that would line up with Shinsouâs age.
Mic walks in to find Aizawa staring at the letter blankly.
âSo? Are you two secret cousins or something?â
Aizawa is silent for several more seconds before he finally speaks.
âWell, the good news is the adoption process just got a whole lot easier.â
