Takao Kasuga is a lonely boy who spends his days immersed in books to escape his frustration with life. His only source of joy is the beautiful Saeki, who he secretly admires from afar. However, Takao's obsession goes too far one day when, in a moment of emotional folly, he steals the girl's gym clothes and takes them home with him. Worse, his terrible deed is spotted by Sawa Nakamura, a mysterious outcast who sits behind him in class who threatens to reveal the boy's secret unless he promises to engage in a contract with her. At first it seems Sawa just wants some companionship, but soon it becomes clear that this "contract" involves more than mere afternoon chats. In fact, Takao is about to discover just how dangerous his bond with Sawa is and how it threatens to tear everything - his life, his love, and even his sanity - apart.
Strange things have been happening at a local high school... mysterious disappearances, strange powers and brutal murders all emerge amongst kids who, up till now, have been perfectly normal. Even the Shinigami (Angel of Death) herself has been sighted. What's happening? The answers lie in the mysterious creature known as Boogiepop...
Although the actual stories aren't overly similar both shows manage to create a dark, unsettling atmosphere with a distinctive visual style. Beyond that Boogiepop Phantom plays around with the idea of people hiding their fears, and bringing them to the surface in the form of isolation, madness and insecurity. Aku no Hana might not explore exactly the same themes or fears, but the similarities between the two in terms of atmosphere, tone and how it makes the viewer feel are definitely there.
Meet the bizarre and twisted psychiatric doctor Ichirou Irabu. Occasionally taking the form of a lime green bear, a young man or even a small child, this freaky physician and his seductively sadistic nurse Mayumi deal with all manner of patients. Though in order to satiate his rampant injection fetish, everyone receives the same treatment: a large vaccination, whether they need it or not! From a trapeze artist suffering from insomnia, to an office worker tormented by a permanent erection, to a romance novelist with OCD and stress-induced vomiting, no one is safe from Dr. Ichirou's unique and psychedelic medical practice.
Both Aku no Hana and Kuuchuu Buranko's unique animation styles mix reality with anime art, albeit in different ways. They have a similar focus on the darker side of human psychology and people's mental states & choices when they're under stress. I think both also have good "shock value" and characters that keep you on your toes guessing what their next move will be.
One night, Madoka has a terrible nightmare – against the backdrop of a desolate landscape, she watches a magical girl battle a terrifying creature, and lose. The next day, the teen's dream becomes reality when the girl – Homura – arrives at Mitakihara Middle School as a transfer student, mysteriously warning Madoka to stay just the way she is. But when she and her best friend Miki are pulled into a twisted illusion world and meet a magical creature named Kyubey, the pair discovers that magical girls are real, and what's more, they can choose to become one. All they must do is sign a contract with Kyubey and agree to fight witches that spread despair to the human world, and in return they will be granted a single wish. However, as Homura's omen suggests, there's far more to becoming a magical girl than Madoka and Miki realize...
Both are about the relationship between a nerdy boy and a strange girl. They both start with the nerdy boy doing something a little weird (preverted) after school hours and the relationship that ensues.
Both series have they're weird quirks and darkish atmospere (although Aku no Hana's was considerably darker). I think both seriesfit well together.
In the streets of Tokyo, a new menace has surfaced: Shounen Bat, a young boy who wears golden roller skates and a baseball cap, and likes to whack people on the head with a golden baseball bat. These seemingly unconnected and random attacks soon become a police investigation... but after all is said and done, is there a pattern to this chaos?
Both of these series feature gorgeous, well-done animation, a tense atmosphere, and deal with characters on the fringe on society. I also feel like both series are easy to identify with, but deal with issues that are a part of Japanese culture.