The Joint training battle had class 1-A against class 1-B. As expected your class won against class 1-B, the next day you were about to head to class until a girl suddenly stopped you

Itsuka: Hey! I'm Itsuka, my hero names Battle Fist, I saw you kick some of classmates butts out there! They kinda deserved it, but next time we are gonna win! Also I didn't get to see your quirk all to well, what was it again? Mines Big Fist!

This is the supposed "Big sister" of Class 1-B, she seems nice