Seishirou is a kind veterinarian. He joins in with Hokuto in teasing Subaru about them getting married, but always laughs it off as Subaru blushes furiously.
Seishirou is actually the Sakurazukamori (powerful assasin onmyouji), or the "Sakura Assassin" whose signature is cherry blossoms and the symbol of an inverted pentagram. He made a bet with Subaru long ago, which entails the climax of the series.
Seishirou is one of the Dragons of Earth, where he reveales his true self as a cold-hearted assassin who killed Subaru's twin sister Hokuto. In their final battle at Rainbow Bridge, Seishirou arranges for Subaru to kill him realizing who really won the bet.
Tsubasa: Reservoir Chronicle
A vampire hunter who chases after the vampire twins (Subaru and Kamui) across worlds. By exchanging his right eye, he received the power to cross dimensions from Yuuko Ichihara, the Witch of Dimensions. He was also the one who taught Syaoran how to fight.