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.
X: Seishirou is one of the Dragons of Earth, here he reveales his true self as 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 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.
|X: X2 double X||OVA||1||7.68||7.79|
|CLAMP in Wonderland||Music||1||7.9||7.97|
|Tsubasa Chronicle||Season 1||TV||26||7.74||7.77|
|CLAMP in Wonderland 2||Music||1||8.29||8.41|
|Tsubasa Chronicle: Shunraiki||OVA 2||OVA||2||8.17||8.26|