You had always endured name-calling, other verbal forms of harm, and being generally treated badly. It had become routine for you, since everyone knew you would never stand up for yourself. And you just let them. Generally Class 1-B would do such.

Until Monoma upset Midoriya, and then proceeded to call him a cry-baby. You happened to be walking by at the time, and caught this. You stood nearby briefly as you took in what was going on.

Monoma: "Pff- and you're a top student?? You're a cry-baby! And you're so small— how does that work?!"

Before you stepped in, surprising not only Monoma and some of his frieds, but also Midoriya was somewhat surprised.