Seishirō (星史郎) in Tsubasa ~RESERVoir CHRoNiCLE~, appears as a mysterious traveler from Syaoran's past who also gains the power to travel between worlds from Yūko Ichihara, the Dimensional Witch. He is currently searching for the vampire twins, Kamui and Subaru. Like his younger brother, he is a treasure hunter, not a vampire hunter like originally believed.
He is a crossover character from CLAMP's Tokyo Babylon and X. His first appearance is in Chapter 42.
In Tokyo Babylon, Seishirou Sakurazukamori is the leader of the assasain clan, Sakurazuka, a rival of the Sumeragi which is a medium clan to which Subaru and Hokuto Sumeragi (his sister) belong. However, Seishiro doesn't want to follow family tradition and likes Subaru. He makes a promise to Subaru when they were young, that if after living together for a year, he felt something for him, he wouldn't kill him, otherwise he would. At Tokyo Babylon's finale, after losing an eye, Seishirō kills Hokuto (who sacrificed herself protecting her brother). Hokuto casts a spell over him: if he tries killing Subaru again, his attack would reflect on himself and he'd die.
Thus it happens in X: Subaru ends up losing an eye and murders Seishirō due to Hokuto's spell. Subaru then receives Seishirō's eye and becomes the new Sakurazukamori.
In Tsubasa ~RESERVoir CHRoNiCLE, Seishirō is looking for the twin vampires, Kamui and Subaru, he made a deal with Yuko so he would travel dimensions, in exchange of his right eye, like in Tokyo Babylon. Kamui later explained that Subaru gave out his blood to the hunter. He also posseses one of Sakura's feathers, which he used to bring Ōto to the reality.
Once on his travels Seishiro found a young Syaoran, sometime after the boy had been adopted by Fujitaka. Syaoran was holding a book for Fujitaka, and running from several thugs who wished to steal it. Seishiro fought the men off, and rather than take the book from the boy, made a deal instead. Syaoran allowed Seishiro to read the book in exchange for learning how to fight. As both were missing an eye, Seishiro imparted techniques to compensate for the loss.
Syaoran and his party find Seishiro in the kingdom of Ōto, where he is using demons to find the I-1 Oni, which is said to be immortal. He believes the I-1 to be Subaru, and when he finds out it was just an NPC played by one of the park's employees, he leaves taking the feather with him. Before leaving, he warns Syaoran that he should become stronger to retrieve the feather when they meet once more.
The party encounter both Seishiro's brother, Fuma, and the vampire twins Seishiro is looking for in Tokyo. The party learns from Fuma that Subaru gave Seishiro some of his blood and that Seishiro is doing fine (which is probably why he's chasing him, because he's unable to survive without the blood of his "E" or prey).
Later he appeared in Nihon Country and discovered that Fai had vampire blood. Initially he thought Fai and his friends knew the location of Subaru. After realizing they didn't, he intended to leave Nihon, but was challenged bySyaoran for the feather. Seishirō remarks how "Syaoran" is like his father, implying that he has met him during his travels, referring to "Syaoran's" true father and not Fujitaka, Clone Syaoran's adoptive father; throughout the battle. In Tsubasa Chapter 223, it was revealed that Clone Syaoran (reincairnated as Syaoran Li) was "Syaoran's" father.
After "Syaoran" unleashed his ultimate spell (all four of his elemental spells combined) on Seishirō, he reclaimed the feather and was sent into the Dream alone to rescue Sakura's soul. However, it seems that Seishirō intended to give the feather to Syaoran; his brother, Fūma, remarks, "You were planning to hand over that feather right from the start, weren't you? To that kid [Syaoran]." Then starts revealing a kinder, nonviolent side of himself to Fai andKurogane. He then starts a conversation with them about how dangerously powerful The Clone is becoming; but not before he tells them that Syaoran is becoming more powerful as well.
He was last seen viewing the battle between The Clone and Syaoran (both, in a sense, former students of his) before leaving to search for Subaru.
Seishirō currently parted from Nihon looking for the twin vampires.
Later chapters in the Clow's Future arc reveal the reason behind Seishiro's knowing of "Syaoran's" father: the Clone is actually "Syaoran"'s father, which explain why and how they met each other.