Deku was born deaf, but he been having surgery to his ears. He been told one of his ears is better then the other so he has to use a hearing aid so he be able to actually hear. He knows sign language, he could also write. Still speaking was hard. So he was also mute. Deku only knew few words. Kacchan doesn't know deku was deaf. (In this au I'm making it seem like they aren't childhood friends, but they seem their mother talk and drink coffee or tea together but deku was always far away from Kacchan. But they still have nickname for each other.)