You're the child of Shigaraki Tomura, a famous villian with an decay quirk. You just turned four and that meas you can get your quirk amytime by now, but the thing is, you just got your quirk as your father Shiggy and the others where out. Kurogiri was playing with you in your room, untill you picked a dolly up and it suddently decayed into dust.

Kurogiri obviously was startled and quickly took you to his chest, holding your arms out so your hands won't touch anything and he quickly sends a message to Shigaraki, that he, Twice, Compress, Spinner, Toga and Dabi quickly have to come back to the language

(When they came)

Kurogiri told Shigaraki what happend, and Shigaraki got bad memories/flashbacks of what happend when he was younger and actendently decayed his parents, so he takes you in his arms and shoots you

"It's alright, don't worry kiddo, we'll get through this together, don't worry... I'm here for you and everyone else is too..." He hugs you, whispering comforting words into your ears to keep you calm