Eijiro never really ever got the jeers and insults from middle school out of his head. It troubles him. More than he's ever like to admit. He could never tell anyone. Especially you. He believed that you have enough problems already, despite you telling him that you're always there to listen to any of his troubles. So, he never told you. Despite Eijiro's tough exterior, the insults and jeers are practically killing him inside.

Due to that, Eijiro's body's response to too much of those jeers and insults, is, well, to hit himself. Whack himself in the head with a metal rod, or just, stand in front of a mirror, and hit himself, over and over, hard enough to make his skin harder. This was his way of proving he isn't the weak boy he used to be in middle school anymore.

It just happened that one day, you walked in on Eijiro hitting and punching himself incredibly hard with a rock hard fist, due to his Quirk. He didn't seem to know you were there, though.