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.
Ever since the Pharmaceutics Law passed, everyone in the country has unlimited access to a dizzying array of mind and mood-altering drugs. A shot of Slumberine helps you sleep, and any emotion you can imagine is just a pump away. Even vacations can be replaced with shots: take one dose, and your mind is bathed in a relaxing hot springs experience. Amidst all this drug-addled merriment is Kabu, a world-weary twenty-something man, with nothing to do but chase his next big high. But one day, after running out of P, his drug of choice, he experiments with a new super-drug, Ultra Heaven, which completely re-writes his sense of reality and changes his life forever...
These two seinen manga are about bored young men, disillusioned with the world. And while one focuses on psychopharmaceuticals, and the other on psychoanalysis, both deal with what's going on in people's heads, and are fairly dark. Besides that, their plots aren't terribly similar, but I think they might appeal to the same audience.
These manga share a common style because how the events are mainly described by the images, since both main characters experience an alteration on the perception of reality (by drugs in Ultra Heaven, by a surgical procedure in Homunculus).
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!
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.
Hikari Hamura, nicknamed Picasso by his peers, always has a pencil and sketchbook in hand. Though he used to spend most of his time with his only friend Chiaki by the river's edge, things changed for the boy forever when a helicopter crashed on top of them, killing Chiaki and leaving him virtually unscathed. Upon awakening, Hikari is told by Chiaki's spirit that he is alive because she prayed for his safety, and in return, he must help classmates in need with the power of his sketchbook, or his body will rot away. Now, with the help of his angelic partner and his artistic skills, Hikari must help save his fellow students, gaining plenty of friends along the way.
The protagonists of Homunculus and Genkaku Picasso both have the ability to see others' true thoughts/selves, and use this ability to help people deal with past trauma and other difficulties.
Genkaku Picasso is a bit like Homunculus-light: the former is much less graphic and usually doesn't deal with quite as heavy themes. However, even though Picasso ran in Shounen Jump, it was written by noted seinen mangaka Usamaru Furuya, so seinen fans will find plenty to love in it.
Both Himitsu and Homunculus are medical/psychological sci-fi manga about delving into people's minds. The former is a forensics manga about watching a dead person's memories, and the latter is about a man who can see people's inner selves. They also both have an element of mystery to them- Himitsu is a crime drama, and the main character in Homunculus is often preoccupied with figuring out why people look certain ways. Lots of great drama, though some readers may find bits of each disturbing.
In an old building, between floors seven and eight, a man has been incarcerated in a locked room for the last ten years. With no idea as to why he was taken in the first place, nor where he is, the man has spent the last decade alone with no contact to the outside world except for the television in the corner. One day, a group of men in dark suits burst into his cell and announce that his sentence is over before stuffing him in a suitcase and dumping him in a park. Free to do as he wishes, the man has just one thing on his mind: revenge against those who ruined his life. Now with the perfectly honed body and a festering anger gained from his time alone, he has become a lethal fighter hell bent on finding out exactly what the reason for his imprisonment was, the people behind it, and making them pay for it.
Both Goto and Nakoshi are men who suddenly found themselves in their thirties without a meaning for what they have being doing with their lives until an important event that made them realize their mistaken attitude towards life. Both are unemployed and living an easylife at the moment while researching for their motto and comparing their current life with their previous.
Revolving in a psychological thriller, these two manga talk too about the personality disorders related to undertaking plastic surgery and how can money influence in peoples lives.