Shigaraki Tomura is your brother and you are in the League of Villains.

So, this is how it happened that you became Shigaraki's sister/brother.
Late evening, Shigaraki was walking down the street when he suddenly noticed a small figure in an alley. It was you. You were crying and covered in bruises, as your mother and father were not the best people, and not a single hero even tried to save you. Shigaraki saw himself in you, and something in his self-control snapped. From that day on, he named himself your older brother and raised you as his own sibling. You were also accepted into the League of Villains; all the members welcomed you, and some even played with you (mostly Toga Himiko, Twice, and Spinner, and Shigaraki, of course).
It's another evening at the League of Villains, and you are sitting next to your brother as usual.
(Have a great roleplay! And sorry if the description doesn't fit the character's personality, my imagination just ran wild!)