Ritsuka's older brother. He was seventeen when he was found murdered, although he died under mysterious circumstances and could only be identified by his dental records.
We later learn that Seimei had faked his death, but who the person that died in his place was has not yet been revealed. He is also the sacrifice of the Beloved pair. Soubi was given to Seimei (mostly at Seimei's request) as a fighter when Seimei was only 14 years old. It is speculated by fans that there is some kind of rivalry between Seimei and Ritsu over Soubi and perhaps over other things, but there's no information in the series that confirms these speculations.
When Ritsuka lost his memory, Seimei stepped in and became an extremely important figure in his life. He protected Ritsuka from their abusive mother, Misaki, and was a great source of comfort and trust. As a result, Ritsuka was hit hardest of all by the news of his brother's death. He is very bitter and vengeful towards those who may have been involved in the killing of his brother.
His personality is subject of great debate, and many readers have very different interpretations of his actions. To make things more confusing, the characters in the story themselves clash with their different beliefs and experiences. Ritsuka, Soubi, and Misaki claim Seimei was a kind, gentle, loving person. In contrast, everyone else has the very opposite idea: they say he was cruel, merciless, and hateful. They claim Seimei acted strangely and would be close to no one, and go so far as refusing to eat in the presence of anyone else. Ritsuka denies all of this; however, in recent chapters published in spring 2006 he has some startling flashbacks that show a more disturbed, obsessive side of Seimei, and even sometimes borderline incestuous behavior towards Ritsuka.
In flashbacks in later manga chapters, it has become increasingly clear that Seimei suffers from mysophobia, a fear of contamination of dirt or germs, although it is largely reader speculation based off his obsessive behaviour about cleanliness and such innocent items such as homemade food being "dirty."
Currently, he and Nisei, the true Beloved fighter, have traveled to Goura and has infiltrated the Academy of Seven Voices. His reasons for this infiltration has not yet been revealed. He betrayed Nisei by refusing to assist him in a two vs. one spell battle with Ritsuka and Soubi. This kind of action is very much consistent with the many negative beliefs that most of the characters in the series have about Seimei.
