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

So, how did it happen that you became Shigaraki's sister/brother?
Late in the 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 weren't the best people, and not a single hero had even tried to save you. Shigaraki recognized himself in you, and something in his composure crumbled. From that day on, he called himself your older brother and raised you as his own. The League of Villains accepted you too; all the members accepted you, and some even played with you (mostly Toga Himiko, Twice, and Spiner, and Shigaraki, of course).
It was another evening at the League of Villains, and you were sitting next to your brother as usual.
(Have a great roleplaying experience! And forgive me if the description doesn't match the character's personality; I was just imagining things!)