Katsuki wasn't only a bully to Izuku, he also bullied Mikumo and even his own twin, Gogo.

But one after another Izuku, Mikumo, and Gogo disappear without a trace. Mitsuki, Masaru, and Ink are all distraught, and at first, Katsuki doesn't care. He thinks that the three of them are just being dramatic and will show back up in a few days.

But they don't. Their case runs cold, no one can find any trace of the three missing kids. Inko falls into a state of depression and, believing her babies are dead, leaves Japan to start a new life somewhere else.

Mitsuki and Masaru lock away every trace of Gogo, they even go so far as to seal his own room shut. They completely ignored he ever existed because its easier than keeping up hope that he's alive when that's probably not the case.

Katsuki bu this point is trying to find something, anything that will lead him to the three of them. He doesn't know why, but them just vanishing like this doesn't feel right, something in his gut tells him its his fault, but he doesn't know why it would be. (Dumbass)

--

Izuku, Mikumo, and Gogo basically start their own villain/vigilante group. They aren't famous except to the underground heroes they help and those guys wont rat them out. Vigilantes are a key part in underground heroics after all.

The three of them recruiter a few people into their small family as they call it: 

Shinsou, a boy that they saved after he ran away from his abusive home life. 

Jiro, She doesn't live with them, butis stilla key part in their operations. She's be of the ones that gather intel.

The Hatsume twins, Mei makes all their gear and security features around the base. Mao is another intel gatherer, his quirk allows him to see through walls so him paired with Jiro makes for an ultimate intel team.

Dabi, their basically parental/older brother figure. He's a mess, but a functional mess.

--

Jiro is the onlyone that goes to UA, everyone else doesn't, but eventually, when she thinks that Aizawa & Nezu are trust worthy enough, she asks the group if she can tell them about them. The group is hesitant at first, but ultimately gives in.

They get taken to UA and are enrolled in a new underground heroucs class, class 1-X (Except Dabi, he is taking collage hero courses via Nezu & Eri is well like six soo yeah)

Katsuki doesn't meet any of them until they have already been in the school for three months. He tries to apologize to them, but the trio isn't having any of it and teaars him apart. (They might forgive him later, but idk)

(that's all I got for now. Leave some suggestions on what you would like to see from this)