If you liked the Angel Sanctuary anime, the Anime-Planet community thinks you'd like:
In the time of the Vietnam War, an American military base in Japan is plagued with a rash of killings whose assailant is unknown. Enter Saya, a mysterious young woman who happens to be the last of the vampire race, and has been charged by her government agency employer to investigate the sinister killings. With demons and creatures abounding, Saya must infiltrate a school to put a stop to the bloodshed -- unless the monsters get to her first...
I have no idea how anyone could actually understand my reasoning for this. Fore extreme violence doth take place. The plots are completely different, but both tended to make me stop and really question about the existence of a divine creator. Both anime also have an almost dirty feel to the animation.
On an island where cherry blossoms bloom all year-round, love seems to always be in the air. It is in this magical atmosphere that Asakura Junnichi lives, and when he dreams he travels to the dreams of others, rather than have any of his own. In everyday high-school life, he is accompanied by his adopted sister, Nemu, and an eclectic group of friends including a j-pop idol-in-the-making and a girl they knew from their childhood. Promises, and magic, and love -- Junnichi seems to dream about every girl he knows, but which girl dreams of him...?
In present day England, a war is being fought. The average citizen does not notice, but vampires are running rampant among the populace, and it's up to the Hellsing organization to contain the bloodsucker threat, using even their own weapons against them. Alucard, a vampire himself, is the best agent the organization has to offer, answering only to Integra Hellsing herself. No one knows much about this mysterious figure fighting against the occult, but things starts to change after he “recruits” a new agent to the establishment...
Both carry a sense of taboo with their story line, playing with religious views and love. For an example, Angel Sanctuary takes a touch of incest (Setsuna loves his sister) and Hellsing is constantly bringing in something to do with religion if not just spirituality in general. Both are great feeds for that more complex, dark side of every anime fan.
Kazuna is a young man with a fairly normal life. He attends high school, lives with his surrogate family, and models for his somber love-interest Yaegashi's paintings. But recently, he has suffered several crippling attacks at the sight of blood -- attacks which leave him incapacitated and out of control. Kazuna must now reunite with his sister who he has not seen in years, and discover the truth behind his family name and vampiric genes, before his bloodthirsty desires destroy himself, or others close to him.
Both Hitsuji no Uta and Angel Sanctuary are dark anime with stunning animation (although Hitsuji's falls short, at times), with a common feeling of tragedy, and an underlying incestuous theme (though this is more evident in Angel Sanctuary).
In Japan, there is a disturbing rumor being spread that people's shadows are ripping from their bodies, never to be seen again. For one young man, the frightening nature of this rumor is just beginning to manifest in the form of a nightmarish world that he can't seem to escape from. In this place full of dark minions and demons, he and his friends fight futilely against unknown enemies, only to be brought back to the "real world" as quickly as they left it. Is this real, or a dream? Only one person holds the key to destroying and saving the world as he knows it...
In both Interlude and Angel Sanctuary there is a similar feel of a slice of life series turning into something alien. In Interlude this alienation resolves around the appearance of the existence of two worlds, while in Angel Sanctuary this alien aspect is the existence of angels. The main difference between the two series is humor. Interlude has a healthy amount of humor, while Angel Sanctuary has near to none. Due to this, you will not find any similar characters as well.