Subaru and Seishioru
The cherry blossoms are beautiful. Do you like sakura?"
When he is about eighteen, Seishirou finds a young Subaru admiring the cherry blossoms. Intrigued by the boy, Seishirou makes a bet with him that when they meet again, they will live by each other for a year, and if in that time, Subaru can make Seishirou feel any real emotion, he will let him live. If not, Seishirou will kill him. Seishirou then seals the bet with his marks, inverted pentragrams, on the back of each of Subaru's hands, and seals away Subaru's memory of the bet as well.
Eight years or so later, the meet again in Ikebukuro train station. Seishirou, masquerading as a friendly vetrinarian, befriends Subaru and his twin sister Hokuto over the course of the next year. As the year draws to a close, Seishirou loses his right eye protecting Subaru, and Subaru realizes through his guilt over this, that he loves Seishirou. However, when he goes to tell him this, Seishirou reveals his true nature and the bet, and tells him that the year is up and that Subaru has lost the bet. Subaru's grandmother saves him before Seishirou kills him, but he goes Within himself to escape. In the end of Tokyo Babylon, Hokuto sacrifices her own life to bring her beloved brother back to reality, which works, but sends Subaru farther down the path to being a very emotionally damaged man.
Nine years later, they meet again, although under slightly different circumstances. The year is 1999, and the fourteen Dragons of Heaven and the Dragons of Earth are fighting to determine the fate of humanity and the world, as is there destiny. As one might guess, Subaru is a Dragon of Heaven, fighting for humanity, and Seishirou for the Dragons of Earth, fighting for earth. They have several on-screen encounters, mostly resulting in fighting, cryptic-ness, and Subaru being the truly fucked-up man he's become. In X, part of their story has an end, and part of it we're still waiting for ^^