@dope69
Almost every character in this series is emotionally abusive in some way.
The mother is abusive in the sense that she neglects the protagonist, saddles him with responsibilities, and does not hold her older son accountable.
The teacher is abusive because she removes the protagonist's agency. She smothers him with what she thinks would be best for him, driving away and sabotaging his relationship with his best friend. She says she wants to take care of the protagonist and will let him go whenever he wants once he is safe, but her pattern of behavior betrays this for a lie. She will strive to make the protagonist reliant on her alone, turning the object of her obsession into her own possession. If you're into that, cool, I get how that could seem nice, but it is unhealthy.
The idol girl pushed the protagonist towards unhealthy coping mechanisms (is suicide a coping mechanism? Probably not, but whatever). I'd argue that her tryst with the protagonist was not super abusive but it was definitely not healthy and neither of them were the mature adults they needed to be to support one another through their depression.
Chako is the only female character we've seen that hasn't been abusive or unhealthy. The only ways she asserted control over the protagonist's life was telling him to not associate with the girl pushing him to suicide, which is objectively a good call that a friend would make. I suspect she is about to self-destruct now that her hopes of leaving town have gone up in smoke, but at this point she is a supporting friend.
Honorable mentions:
Gen is abusive for similar reasons to the teacher, since he wants to trap the protagonist in the town forever. I think he's a personification of the town in many ways, and his unhealthy relationship with the protag is emblematic of the grip the town itself has on him.
Author dude is abusive because of how he pushes the people around him into unhealthy and self destructive behaviors. The idol wanted to kill herself because of him, and it's already been shown that all he needed to do to get her on track was say "hey go do your work again". He clearly has some baggage with the mother. He also is leading on Chako, an underage teen girl, towards a sexual relationship, which is pretty fucked up.
Finally, the protagonist himself is a fuckhead, mostly due to the way he's been treating Chako. Man needed to come clean with his secrets to her, since it's clear she trusted him with hers. I get he doesn't want her to know he's slept around, but they're in a situation where secrets are causing problems.