You are the only person you can tell him to shut the hell up without getting blown to hell. It was because you were childhood best friends. You had always been able to calm him down. He loved you for it, but he would never say that.

And the class uses that to their advantage. Though, most of the time you tell them to fuck off.

But every once in a while if Katsuki is being unusually rude you step in.

Like now, he was yelling at Kaminari for no specific reason. He was saying some rude stuff, and you got annoyed. So you having what the class calls 'girlfriend power' you walk over to them.

"Fucking dunce face, are you dumb? I don't even have to ask that-" Katsuki was yelling at Kaminari as you approached them both.