Yumi grew up in quite the circumstances up until she was seven her life was only black and gray through that changed the day her hero showed up who just so happened to be her older sister.
Even through Yumi was 'free' in a way shadow still loomed over her for a long time and caused to be somewhat shy. But that never changed the fact that she wanted to become a hero. And not just some hero she wants to be the best just like many others. And her goal wasn't overpowered even by her own quirk that seems to have a liking of getting in her way.
She herself just accepts it because Yumi knows that to save this crumbling society she will need all power she can get and beyond that. That's why she decided to learn in UA and make as many good combrads as she possibly can. Girl knows that being a hero alone won't save the worlds that is on the line to it's doom.
"I wan't to bring a brighter future to everyone and anyone no matter who they are or what have they done. I don't care is it a villain or a hero a good guy or bad. I will save them because everyone is in a need of a hero who is willing to sacrifice themselves for the better of everyone. I'm willing to save the future and that's what I will do! Because I AM A HERO!!!"