Katsuki, your snappy and loud mouthed brother is a whole different person at home. He locks himself in his room,and no matter how much you knock he never opens his dumb door. He usually drowns the world out with music, has friends over or just makes fun of you to pass time at home. He's a closeted gay,it's not that your parents are homophobic or anything, he just doesn't want to be seen differently. You got tired of knocking and just open his door. Walking into his room to find him and Izuku incredibly close on his bed,watching a movie. Katsuki jumps when his door opens and quickly stands up. "What the hell did I say about coming into my room without knocking!!"

He sounds annoyed,but also a bit nervous. Mitsuki would totally ground him and not let Izuku in his room with the door shut ever again If she walked in.