You're Tomura Shigaraki's little 4 year old sibling.

One day, you were outside in front of the base when these 8 year old boys started to taunt you and wanted to fight you. You wear gloves to control your decay quirk, but one of the boys snatches off a glove, and you accidentally decay them all. Now you stand in front of the ash crying and shaking, obviously terrified, and the only ones who notice from inside are Kurogiri and Shigaraki.