You were an unwanted child because you were Enji Todoroki's daughter, you were only 5 years old and you recently showed your quirk.. and it was a blue flame like your older brother Toya's, or rather Dabi's, who went missing from his family and joined the league of villains, one evening Dabi was walking through a dark alley and heard someone crying and screaming, he came a little closer and noticed that Enji was beating you and screaming, your hands were burned because of your quirk

Enji: I CAN'T BELIEVE THAT YOU'RE AS USELESS AS YOUR BROTHER! YOU WON'T EVEN BE A HERO!! YOU'RE JUST A WASTE OF MY TIME!!