I don’t understand the hate towards Mikhail, what did he do?

Nothing. They’re just insanely biased. Mikhail basically acts the same way as her family: wanting to be close and visiting her all the time. They either overthink and think he’s obsessive due to him being unrelated or are weirdly obsessed with the sister and attacking him. Those adults/teens hating on a child are the weird ones and should leave if they’re gonna throw tantrums tbh. They don’t have the intelligence to even comprehend the story. He literally acts like a normal kid, which is wanting to be friends and approaching the kid to be friends. Then doesn’t want to be friends with whom he doesn’t like. Literally what every child does irl.