ritsu doesn't like or dislike shu, he has a normal colleague relationship with him where they aren't particularly close but don't hate each other either (you could say ritsu felt annoyed by shu's existence at first like with the rest of the oddballs who "stole" his brother from him, but that's past them, he doesn't think like that anymore). the reason why he calls shu "oshisan"-san is just to be annoying lol taking mika's appellation for him and adding another "san" for the funsies the same way he calls makoto by "yuu-kun" because of izumi or hokuto by "hokke-kun" because of subaru. he doesn't hate any of them, makoto even says that he and ritsu became close now and could be considered friends, so he really just does this bc (again) he's a little shit kdjsnfjkdg