Adaptation Displacement: Many think that like other 7th Expansion works, the manga is adapted from the VN. It's the opposite. The VN is simply taken as reference more often since it's considered better than the manga overall.
Manga: Yoshihito Kanamori from "Mesomeso-san" (the first chapter) started as the homeroom teacher of Marie Moriya, a young girl bullied by her classmates. He used to defend her from the bullies, until the stress of his job got to him and he started to take his stress out on Marie. His supplementary lessons include molesting her and putting it on camera. When Marie gathered up enough courage to stand up to him, he strangles her to death, singing her a song about graduation from life to death while doing so, then dumps her corpse in the old school's septic tank. He also considered murdering other children when he thought that they were eavesdropping. When he encounters Marie's spirit in the restroom, he gives her a long "The Reason You Suck" Speech about her not going to the police to report on him while she was still alive and called her an imbecile who couldn't fend for herself. He then opens the door to one of the bathroom stalls, and upon finding that Marie has apparently physically manifested, he tries to strangle Marie a second time. When the deed was seemingly done, he proceeds to rape the corpse when he notices it was still there. While he claims that he did all this out of love for Marie, it's ultimately proven hollow.
Visual Novel: Nafumi Shintani from Renoir of the Art Room was an art room teacher who seemed to be a Cool Teacher, given her popularity among the young male students. In reality, Nafumi was a prolific child molester who got off on the suffering of young boys, deeming it as beautiful. To this end, she invites young boys into the art room, and she would offer them cups of drug-laced tea, after which she would proceed to torture their unconscious bodies with cigarettes and palette knives. In addition to this, she had dozens of sketches of abused boys in her desk. One of her past victims, Yuuki Noda, loved his teacher to the point that he allowed her to abuse him if it meant his feelings for her were returned. Eventually, Yuuki tried to escape her, but she grabbed his shoulder, which caused him to trip and fall underneath a table, resulting in a heavy bust falling on his head. A week later, she drugged and attempted to torture another boy, but she was stopped by the titular Renoir of the Art Room (who was really Higanbana filling in for the real Renoir). She expressed zero remorse for being indirectly responsible for Yuuki's death, stating that he was a "stupid kid" who deserved to die. Depraved and perverted, Shintani repeatedly showed that she cared only for herself and her lusts.
Ensemble Darkhorse: Midori seems to be the most popular character after Higanbana and Marie, having amassed quite a bit of fanart.
Jerkass Woobie: Midori Kusonuki, a Rich Bitch who used to flaunt her wealth to the other students and got bullied for it. It's understandable that they wouldn't appreciate her acting haughty, but how is said bullying carried out? She's beaten just about every day and at almost any given moment by her classmates, and she's been forced to eat curry full of dead bugs and cockroaches. And what does she do in response to this? Escape into a fantasy world where everybody loves her. Compared to the likes of Hikaru, whose response to the bullying he brought upon himself was to torture the school's rabbits before finally killing his tormentors, Midori ultimately comes across as a sad, pathetic young girl whose whole life has turned into a spree of Disproportionate Retribution so bad she'd rather let her soul be eaten than endure it.
Les Yay: Between Marie and Higanbana. In most of her talks with Marie, there would be a heart at the end of the speech. From this, you almost get the impression that she is flirting with her.
There's also some slight Ho Yay between Hameln and Hikaru in "Hameln's Castanets", when the narrative mentions that when Hameln leans towards Hikaru they're "close enough to kiss".