Ichijou Mashiro seems to have it all - he's popular, good looking and attends a prestigious prep school. But Mashiro has a secret - he's neither male nor female. So far, Mashiro's been able to live his life as a boy, but all this changes when he's informed of a new class he must take in order to graduate. Mashiro is told to find the Key to graduate, and the only way to do so is to enter a nightmarish world with several other anonymous students in a twisted group therapy session. These terrifying after school classes will challenge Mashiro's notions of friend and foe as his body and soul are put at the mercy of the worst kind of enemies: his classmates!
Fifteen-year-old Tsubasa's parents divorced three years ago, separating her from her beloved twin sister Arisa. However, they've communicated secretly with letters and are now meeting with each other for the first time since the event. Arisa is everything Tsubasa is not - she's popular, gentle, has good grades and has a new boyfriend, while Tsubasa can't seem to find girl friends to be cutesy with and has earned a reputation of being a delinquent leader. The two decide to switch places for a day and Tsubasa is enamored with Arisa's school life. But beneath the surface of Arisa's life lies dark secrets, not only about her classmates, but also Arisa herself. Tsubasa must infiltrate her beloved sister's life in order to discover the truth behind shocking events taking place...
Both After School Nightmare and Arisa have a rather 'psychological' component to them. They both have deal with the workings of the human mind, and how decisions can affect those around you and your own current mental state. (Also, they both share the same 'school-life' feature and includes twins...)
They have their differences, though. Arisa focuses more on the darker part of the mind, while After School Nightmare was more about inner battles. I would say they were both equally dark though. Where Arisa lacks the physical fighting is makes up in extreme bullying and unexpected twists. Where After School Nightmare lacks twisted minds, it makes up in dark drama and intriguing, unique characters.
Susamu Nakoshi, a man disillusioned with life, meets Manabu Ito, a young medical student bored with life. To relieve his boredom Ito offers Nakoshi 700,000 yen to undergo a trepanning to see if it will unlock his sixth sense, and Nakoshi eventually agrees. However, much to his dismay and chagrin it actually succeeds, and now Nakoshi must contend with seeing homunculi, the metaphysical and symbolic representations of a person's psychological disorder. He now helps people, usually against his own will and often theirs as well, with resolving their psychological problems.
Another manga in which the main character can sometimes see others as they truely are. Instead of appearing like a normal human, characters take the form of sand monsters, black-clad knights, giraffes, diembodied arms, and young children dressed as robots. Both are fairly dark, as most of these deformities were caused by past trauma, or psychological instability.
Note: I enjoyed Homunculus far more, as ASN spent way too much time wallowing in teen angst.
Gender is really important in both After School Nightmare and Sengoku Nights, because there's a character in each who isn't fully either. There's also a guy who's trying to force himself upon the protagonist, much to their dismay. The circumstances are a bit different, but fans of complicated relationships or even the misfortune of the main character, should enjoy these series. :)
I realize this is a pretty strange recommendation, but stick with me for a second... Though the plots are very different and After School Nightmare has a more fantastical premise- if you particularly enjoyed the romance aspect of one of these stories I think you will find the 'forbidden' feeling in other to be quite similar.