Seishirou befriended Subaru and Hokuto, leading Subaru to believe that he cared for him—Subaru inevitably fell in love with the kind veterinarian he believed Seishirou to be. Seishirou was everything that the painfully shy, blushing sixteen-year-old might have been attracted to in a man; during the course of the year, Seishirou even lost his eye protecting Subaru. Believing that his affections might be returned, Subaru finally worked up the courage to confess his love for Seishirou—only to have his arm broken and his heart completely shattered. The truth came out: under the sakura tree, Seishirou had made a Bet with the child Subaru. Seishirou would stay with Subaru for one full year; if he felt something for the younger boy at the end of the year, he would spare Subaru's life. Seishirou, saying that he felt nothing for Subaru, left the boy to his heartbreak.
Hokuto, hardly one to stand by in such a situation, willingly confronted Seishirou beneath the sakura tree—she was killed by his hand, but not before casting one final spell. Should Seishirou ever attempt to kill Subaru in the way he killed Hokuto, the killing blow would backfire, and Seishirou himself would die. Subaru was brought out of his catatonic state by his sister's death, and he vowed to avenge her, resolving to forget and eradicate any trace that he had ever loved Seishirou.
Nine years passed, with Subaru growing graver and becoming busier with onmyouji work. His strength, already formidable as a teenager, has increased tremendously, but he is still unable to defeat Seishirou, whom he has been pursuing for the past years. He has trained in order to present himself as enough of a threat to Seishirou; although he says that he wants revenge, in truth Subaru wants Seishirou to kill him. As the year is 1999, Subaru is involved with the Dragons of Heaven in protecting humanity from destruction. He fights primarily against Seishirou, and his magical power is surpassed only by the two Gemini. Subaru develops a close friendship with his Kamui; he sees himself in the teenager, and it is Subaru who rescues Kamui from the depths of his own catatonic unconscious when he goes 'within'.
Subaru's world, changed irrevocably at Hokuto's death, is once more turned upside down upon Rainbow Bridge. The two onmyouji are engaged in battle; Subaru raises his kekkai, but the other Dragons of Heaven are not present to aid him. Seishirou uses the same killing move on Subaru as he had used to kill Hokuto, and the dead twin's spell takes effect—the battle ends with Subaru's hand through Seishirou's heart. Stunned and completely unable to do anything more than listen numbly as Seishirou whispers his dying words, Subaru remains clutching at Seishirou's dead body even as his kekkai crumples around him. Having lost the person he most wishes to protect, Subaru cannot create kekkai and is useless as a Dragon of Heaven—Kamui takes him away, but Subaru once again withdraws into his inner mind.
At present, Subaru has also taken on the responsibilities of the Sakurazukamori, as successors kill the previous guardians in order to inherit their position. Seishirou's successor was destined to be the person whom he loved most, and it is only the knowledge that he truly meant something to Seishirou that keeps Subaru alive. The inverted pentagrams on his hands have faded, and he has brought balance of sorts to the spiritual state of Japan—he is the new Sakurazukamori, but he remains the most powerful head the Sumeragi clan has ever seen.