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 murder 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 claims Seimei was a kind, gentle, loving person that always protected him. Soubi, on the other hand, is almost afraid and spiteful of Seimei. He tells Youji he would have to wait a hundred years before Seimei would speak to him. Misaki, Ritsuka and Seimei's mother, believes Seimei will protect her from hurting Ritsuka. Others, such as Nagisa and 7, think 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. Soubi's friend Kio bitterly resents Seimei for the apparently abusive manner in which he treated Soubi, but Soubi refuses to back up Kio's accusations. Ritsuka denies all of this; however, he has some startling flashbacks that show a more disturbed, obsessive side of Seimei, and even sometimes borderline incestuous behavior towards Ritsuka. Seimei has said in Volume 7 that in his world, he and Ritsuka are the only two humans that exist and everyone else to him is an animal. He later states in volume 8 that Ritsuka is the only human alive that is exactly like him. When he meets Ritsuka again he asks Ritsuka if Ritsuka would still love him. He states that if Ritsuka loves him so much that Ritsuka would be able to give his whole being to him, his body, soul, love, and mind.
However, it has recently been confirmed that Seimei does indeed have a violent personality, or destructive behavior at the very least. In volume eight, Seimei stabs the wary Ritsu in the eyes after the two have a short yet heated conversation about Soubi; the wounds leave Ritsu blind. Seimei then proceeded to write the haunting message "Ritsuka I'm Back," on the wall in Ritsu's blood for his brother, Ritsuka, to later see. There are also suggestions in canon that Seimei possesses powers beyond that of a normal sacrifice. He has been described as inhuman by Soubi and others.