Tokyo Babylon is a modern-day fantasy manga, written and drawn by CLAMP; it is based in Tokyo(Suprise!). It follows motifs of Christianity and myths of Japanese magic, Shinto.
The main character is a young boy Subaru Sumeragi, an innocent boy who is the 13th head of his clan. However, he misses his classes quite frequently, due to the fact that he is a medium. A person that can talk to spirits. His clan's heads have been able to use strong magic for generations and he is the newest leader. It follows his missions as a medium, being hired by corporations and the government, who have used the Sumeragi clan for generations.
Subaru also has a twin sister, Hokuto Sumeragi. She is quite the opposite of her brother. She's energetic, outgoing, and unable to use most magic. But she is the one who keeps him going, and is the bind between Subaru and Seishirou.
Seishirou Sakurazuka seems to be a gentle, kind, intelligent veternarian. However, he is actually the last head of the great Sakurazuka clan, a clan of assasins. But to use the plural is incorect, for at any given time there is only one. To become the assasin, named Sakurazukamori(Guardian of the Cherry Blossom Tree), one must kill the current head of the clan. Just like the Sumeragi clan, the Sakurazuka clan is also secretly used by the government and corporations. They are the enemy of the Sumeragi clan, where the Sumeragi use defensive magic, the Sakurazuka use offensive. The Sumeragi are Ying as the Sakurazuka are Yang.
But Seishirou stays in the presence of Subaru, being kind and protective, for one reason. Subaru as a child witnissed Seishirou during a kill, and Sakurazuka rules state that such a person should be eliminated immediatly. However, Seishirou found Subaru's innoncence fascinating and made a bet: When they met again, Seishirou would stay with Subaru, protecting him, and if Seishirou did not fall in love with Subaru within a year, he would kill Subaru.
In my opinion this is one of CLAMP's greatest works to date and like most of CLAMP's work it has no problems with having a homosexual relationship. It uses age-old stories, and makes them more realistic. The series is twelve volumes in total, but sadly is not availible translated in English at the moment.
However, the sequel to the series is availible in English. X/1999, known as simply as X in Japan, is a continuation from the events in Tokyo Babylon. But the characters from Tokyo Babylon are not the main charcters.
Tokyo Babylon also has an ova anime availible in English as well. The series only lasted two volumes, containing one episode each. These episodes are not directly based on any of the stories from the manga.
The first is about a woman seeking revenge for her husband, who was killed by one of his fellow partners. This partner seems to be incredibly lucky, as if when he was born spirits watched over him...
The second is about a serial killer, who murders people in trains. It also has a postcognitive, who helps profile serial killers. She finds out that the boy who is killing people, keeps on talking about a game...