In CLAMP's Tokyo Babylon, Subaru Sumeragi, is Hokuto's twin brother (leading to an assumption that his "twin" would be Hokuto and not Kamui in Tsubasa) and also the leader of the onmyoujis clan, Sumeragi, and an expert in exorcism rituals.
Seishirou Sakurazuka is the leader of the assassin clanSakurazuka, who is faking his love for Subaru: he made a promise that "if after living together for one year, if he'd feel any affection towards him, Seishirou wouldn't kill him; otherwise he'd kill him". After the time was up, and after losing an eye for Subaru, Seishirou kills Hokuto (who dies for protecting her brother). Hokuto, in turn, casts an spell over him so that if Seishirou would try killing him again his attack would be reflected over him.In CLAMP's X, Subaru is one of the 7 Dragons of Heaven, while Seishirou is one of the 7 Dragons of Earth. Eventually in the final fight, Seishirou attacks Subaru and Hokuto's spell activates. Seishirou then is stabbed by the Sakurazukamori and dies. Thus, Subaru becomes a Dragon of Earth.
In Tsubasa ~RESERVoir CHRoNiCLE~ , Subaru is also the kind and good-hearted boy as in his original series, though like Fuma says, he becomes terrible if he is angered. Little is known about his relationship with Seishirou, although that from Kamui's words we could assume that as a vampire, Subaru offered his blood to Seishirou and became his "E" or prey; thus the hunter needs to drink his blood in order to survive. This is why Subaru and Kamui are running away from Seishirou and traveling across different worlds to survive. When they reached Tokyo, Subaru is dragged by the power of the feathers in the bottom of the reservoir in the Government Building, and is forced to sleep until someone can release the feathers (though Kamui is unaware of this and limits himself to protect the "shell" where Subaru rests.) While being in this state, he talks to Sakura who is in his same state, urging her to wake up before she is lost to her dreams.
After the revelations with the Clone Syaoran, Subaru is released at last. With the feathers' dissapearance the water dissapears along with it, and Subaru takes the blame, feeling guilty for what happened and offers himself to help. Kamui doesn't agree, stating that "he'll never give Subaru's blood to someone else ever again" and offers his blood to Fai so he can survive in exchange of the egg that Sakura retrieved for Yuko, and granting their wish of getting the water back (which was retrieved by Watanuki and Doumeki in xXxHoLiC, in the house with the dead woman and the egg (which splits into 2 eggs because of "the two futures") is given to Watanuki and Doumeki as "thanks" for them getting water). Kamui willingly gives his blood and Fai is turned into a Vampire, saving his life.
When Subaru watches Sakura and the others, he realizes that they are part of the "event" that Yuko told them about when she granted them the power to cross dimensions. When the group leaves Tokyo, Subaru and Kamui part as well to their next destiny, before Seishirou reaches them.
Yūko Ichihara mentions knowing an Onmyoji with a twin sister whom she has known since the onmyoji was very young. She hopes that the boy will be happy, but there are many definitions for happiness. The Onmyoji turns out be Subaru, as explained in the extra pages at the end of the book.