The Beast

Kaen Kemono.
A 7-year-old quirkless boy of two pro heroes who sold him off to a group of people that experiment with giving quirks to people like All For One.

When he was given a mutant quirk by these people, they keep him locked up so that he teaches himself to use it. When he is successful in his self-training after 5 years, they let him out. But he takes that oppurtunity to kill them and escape.

11 years later, he is recruited by the League of Villains after Dabi finds him and takes him after refusing to go with him. He joins after hearing what Shigaraki has to say.

He will work with them in exchange for freedom.