Nene is a cunt (possibly because of something that Senpai said or did.) Deliberately going out of her way to treat Amane like trash is nothing but distasteful. Even if Nenes being pretty controlling, Kou should grow a pair and give Amane some respect or space. The bystander act doesnt work when you're at the center of it all, knowing you're actions are hurting someone you 'care' about.
It's a very interesting premise and a lot of striking visuals but a lot of the story seems like it could be solved with heart to heart, but that's probably not gonna happen until the teacher steps in because Amane's suicidal.
TLDR: Amane deserves better friends.
Most love problems can be solved like that, but IRL people tend to hide and suppress their feelings. Also they're teenagers, that's even worse. Just look at Kou, she's in this mess cause she clearly can't say no.