So I was in the shower, and this thought suddenly came to me. I thought it would be really interesting to explore a role reversal au, specifically with how Katsuki and Izuku’s interactions with each other would change. So, don’t mind me while I ramble for a sec.
Imagine for a second, that things had been different. That Izuku had been the one to form a quirk while Katsuki was quirkless. How do you think that would change them?
For me, I think Izuku would be a force to be reckoned with. He’s already got such will and determination even without a quirk, so imagine if he had one from the start, no anxiety or self doubt (or trouble controlling OFA) to get in his way. Holy fuck, he’d be a monster!
Yet, I don’t think he’d be quite so... brash as Katsuki is in canon. Self-assured without a doubt. But Izuku would still be that nerdy boy who writes notes about heroes to prepare him for his future. He’d still be that kind boy who wants to be the best hero so that he can save everyone. Just with some added spice. Let us not forget that a person is largely the result of their environment, who they grow up around, and Inko Midoriya is still his mother. I know she raised that boy right, quirk or not.
I think Katsuki, on the other hand, would be the most changed from this. I think at the start, he’d want to hate Izuku for getting a quirk while he didn’t, and maybe it would put a bit of a dent in their friendship. But I think he’d get over it when he realizes Izuku isn’t mocking him or looking down on him for being quirkless, he’s showing how much he still cares about Katsuki. Always encouraging him, telling him that he could still be a hero, even without a quirk(If he believed that of himself in canon, why wouldn’t he believe it here?) Always protecting him from the assholes who were looking down on him. I just know Izuku would be protective of Katsuki, always standing up for him when needed. And afterwards reassuring him, telling him that whatever those mean guys said, it wasn’t true. And even though Katsuki would brush it off, telling Izuku that he didn’t need his help, he’d secretly appreciate it every time. And Izuku would know that, too.
Because of this, I don’t think Katsuki would have such intense self doubt from his situation as Izuku does in canon, not when he has such a perfect hype man who’s always got his back. Maybe he’d be a little standoffish still, hiding his emotions and keeping a wall up like he does, but he’d have a lot less pent up anger, for sure.
When it comes down to it, I think they’d grow incredibly close. When Katsuki receives One For All from All Might, I think Izuku would be the first and only person he tells. Despite being warned not to, he tells Izuku. And Izuku doesn’t think he’s weak for it, or that he got it “the easy way.” He’s excited for him!! Obviously he thinks it’s incredible that Katsuki is training under their idol! And despite being told that no one could know that he knows, not even All Might, Izuku does accidentally let it slip while speaking with All Might one day. Or, more like, he was muttering under his breath and let it out then, because he has no filter when he’s muttering the way he does. And Katsuki did give him a bit of a tongue lashing for it.
Of course, they’d still be rivals. The two of them are determined to be number one, and they both know it can only be one of them. But even more then that, they’re friends till the end, with a bond unlike any other. They’re an unstoppable team. Anyone who knows them knows that you do not mess with Izuku and Katsuki, especially when they’re working together.
TLDR: While I think their core personalities wouldn’t change too much, I think their relationship with he each other would. Katsuki wouldn’t have any resentment for Izuku, and Izuku would still care deeply about his childhood friend, so their friendship would only get stronger as the years go by, making them an unstoppable hero team.