Thunder Brothers as the Kamado siblings is very cool to me but it's even cooler if Kaigaku still doesn't like him at the beginning of the story and grows more fond of Zenitsu as time passes. Maybe when he was interrogated by the hashiras, he agreeded to let Zenitsu be killed as long as it meant being free and someone else had to make the effort to keep him alive
Kaigaku never paid attention to it before but slowly notices how Zenitsu is not the same scaredy cat when they're together and is instead friendly and expressive (in other ways other than crying). He's all sweet to Tamayo and actually stands up to Uzui sometimes and is always watching other people training at night. So he decides to also get those reactions out of him but is so sudden that Zenitsu only gets more suspicious and they go back to fighting
I don't think Kaigaku would ever see him as an equal or superior, but atleast it'd go from "I need this useless brother out of my way" to "I'm the stronger one so I clearly have to care for us both". He doesn't believe in Zenitsu's strength but others (Gyomei cough) frequently help him shift this superiority complex into something that could help them get closer without affecting his ego or hurting Zenitsu's feelings on the way. They also help Zenitsu himself by letting go of the idea of making his older brother proud and instead learn things for himself and his gramps
There's some specific scenes that would have to change to keep it going, like with Gyutaro and Daki. Kaigaku is not kind enough to stop them from hating each other post death over empathetic reasons, but he would point out how useless it is to lie now, and how in the end everyone clearly sees it past you when you're desperate and with the guard down like they are. Yes he is projecting, but if it works it works
