@gunfr0st so this is kind of how I vaguely imagine it would go down.
The very first time Katsuki hears the ringing in his ears, he is a small child. He has just manifested his quirk, the very first explosion which simulaneously terrifies and thrills him. He is young, the explosion is loud, there is an odd ringing sound. He assumes that it is just part of his quirk. They just go hand in hand. This hypothesis of his, (I'm not making assumptions mom! They are educated guess's!) Is proven the more he uses his quirk. Every single time the odd ringing noise is there. He doesn't realize it at first but over the years it slowly gets louder, and lasts longer. The first time he hears it not accompanied by his quirk he is ten years old and sitting at his desk doing homework. His head snaps up and he glances around looking for the source. He knows it can't be his quirk, he hasn't used it. Searching for the source doesn't help. It fades after a bit though and he doesn't concern himself with it again. Not long after, he is in class, he is listening to Deku babble on and on about their most recent lesson. He falls silent for a moment, and the ringing starts. Katsuki hadn't really been paying attention to the other focused on his work, he jerks his head up looking around for the source. It is really loud. Deku is talking again but the words do not make sense. The ringing seems to get louder. In anger he shoves Deku down screaming at the other child to speak louder or shut up. Later at home he will know that was the wrong thing to do. The ringing will continue periodically over the next year. He learns to live with it. He isn't happy about it, but it is just there. He asks his friends sometimes if they can hear it but when they look at him like he is crazy, he never brings it up again. He has to be perfect. He has to. He doesn't mention it again. He trains with his quirk constantly. He knows what his goals are in life. He knows what he wants and how to get it. So he works hard. But it's difficult. The ringing is often now, and he is fighting with his mother more. He learns to cope. Right before he turns 11 he breaks down, it is loud, it is near constant he doesn't understand! His mother is yelling at him about something but he can't understand it. He screams, screams at her to make the ringing stop. She stops. She doesn't know what he is talking about. He is not done being angry. This lands him at the doctors office again. They ask so many questions. He doesn't want to be here. It's dumb. But he wants the ringing to stop.
They call it Tinnitus. They say they need to run more tests. He doesn't want them. Nothing is wrong with him. Eventually though they have an answer. Not one that Katsuki likes. One that he will avoid and pretend isn't happening to him. They do some therapies that are supposed to make it easier to deal with. They tell him they can't make it go away. He is furious. But he learns to live with it. It still bothers him, in class, at home, it can happen at anytime. He does his best but now that he knows why it happens, it doesn't make him as angry. He isn't crazy. He starts using Ear protection when practicing but knows this isn't a great solution. At least in his mind it isn't. By the time that he is in Highschool, he is wearing hearing aids. Now when his Tinnitus starts acting up he can pull his phone out and load up the smart relief Tinnitus app, it bluetooths to his hearing aids and it helps. Sometimes there are bad days but he handles it better now. He has to. He will never allow this to limit him or prevent him from being the very best.
Meeting Present Mic helps in a way that he doesn't know that it could. To meet a Hero who wears his hearing aids, who doesn't hide his hearing loss even if he doesn't shout it from the rooftops. Present Mic will never be his favorite person in general. But there will always be that odd bond. As he makes friends, proper friends, he can trust them, normally just Kirishima though. Sometimes if the ringing is bad and he can't make it stop, its grating on his nerves terrible, he can tell Kirishima, kirishima will just grin and start chatting to the other. Talking to distract him. To keep his mind off of it and hopefully lessen the effects of it. Katsuki knows it will never go away. But he also knows that he will live. He can manage. One step at a time.
