Subaru was born in Kyoto on 19 February 1974, the younger of a pair of fraternal twins, his older twin being a girl named Hokuto. Their parents (who are never seen) were members of the Sumeragi, a clan of onmyouji who have served Japan and its emperors for hundreds of years. Subaru was trained from an early age to become the 13th head of the Sumeragi clan, replacing his grandmother. At the age of nine, he was separated from his sister Hokuto for a year of intensive training in Tokyo. On the day he ascended to the head of the clan, Subaru encountered a strange young man under a cherry tree. The meeting would subsequently be expunged from his memory. Subaru's grandmother ordered him to wear gloves from that day onwards, and never to allow anyone else to see his hands. The old onmyouji cast a spell that would repel any attempts by others to remove them. Subaru never removed his gloves, and was known to wear them even when sleeping.
By 1991, Subaru had grown into a cheerful yet bashful teenager, utterly devoted to helping others -- both in his capacity as Japan's most powerful onmyouji and in "normal" life. As well as exorcizing demons, helping lost souls ascend to the afterlife, entering the minds of comatose people to bring them back to the world, and fighting those who abused spiritual powers, he would often also stand up to bullies and provide comfort and friendship to their victims.
Subaru lives in an apartment in Tokyo, in the same building as his sister Hokuto, whose personality had become the polar opposite to his own -- eccentric and outgoing. Hokuto is vexed by her brother's selflessness, encouraging him to think of himself once in a while, rather than placing the good of others above himself. Although she encourages him to be cheerful and polite, she also criticizes his excessive deference to others, including his habit of bowing to people on the phone "like a salaryman". Subaru wears effeminate costumes designed by his sister, her own outfits being even more outlandish.
Subaru and Hokuto are good friends with a kindly and magically-aware veterinarian named Seishirou Sakurazuka, who they met at a railway station after Subaru's shikigami led him on a bizarre chase. Seishirou acts as guardian to both and claims to love Subaru romantically. Subaru is frequently embarrassed by these claims, as well as Hokuto's gleeful attempts to bring the two together, but gradually grows to accept the older man's apparent love.
During Tokyo Babylon, Subaru has dreams of the day he became the head of the Sumeragi clan: he recalls the strange meeting beneath a cherry tree unseasonably in bloom, with a young man who told him the Japanese folk tale that every cherry tree has a corpse buried beneath it, whose blood dyes the white blossoms pink. When the 9-year-old Subaru asked if the buried people were in pain, the young man, initially confused but becoming oddly amused, made a bet with him, the only words of which he heard were: "If ever we meet again..." and "...but today... I will let you go." Recalling this dream, Subaru begins to wonder if the sinister young man is actually Seishirou, despite the different personality shown between the two. He also ponders Hokuto's suggestions that Seishirou's surname links him to the Sakurazukamori, a ruthless magical assassin clan, but dismisses it due to Seishirou's kind nature.
After Seishirou loses the sight in his right eye protecting Subaru, the onmyoji realizes he loves the man. Unfortunately for Subaru, Seishirou reveals that he is indeed the Sakurazukamori, raised to be incapable of either love or hate. Lifting the veil he had placed on the boy's memory, Seishirou states the nature of the bet he had made: that if they met again, he would live with the boy for a year to see if his heart could be moved. However, the assassin claims he was as barren of emotion as ever, and attempts to kill Subaru, only for the boy's grandmother to rescue him at great personal cost. Subaru becomes catatonic as a result, but recovers when, telepathically, he senses Hokuto's self-sacrifice to the Sakurazukamori.
Subaru decides at this point to drop out of school so that he can focus on training. He continues his work as an onmyouji, remaining compassionate to others but acting more reserved; he also takes up smoking, like Seishirou before him.
By the time of X/1999, Subaru is his twenties, continuing his work as an onmyouji under direction from his grandmother. Although he is recognized to be one of the Dragons of Heaven, and thus capable of creating a kekkai (spiritual barrier) - star-shaped in his case - to protect others during his battles, he has no interest in the fate of humanity; his only concern is to settle his score with Seishirou. However, with persuasion from Sorata Arisugawa and Arashi Kishū, he joins the other members of the group in Tokyo. One of his first acts is to bring Kamui Shirou out of the catatonia into which he had fallen after seeing his childhood sweetheart, Kotori Monou, killed by her brother Fūma. Subaru and Kamui develop a close friendship based on their similar hardships, with Subaru acting as a mentor to Kamui during his further trials. Series creators Clamp, jokingly, refer to Kamui and Subaru's relationship as "sisterly".
Subaru faces Seishirou several times, but each time, though defeated, is allowed to live; he begin to imagine that the assassin does not consider him "worth" killing. When Subaru faces Fūma in battle, he sees the younger man as Seishirou, and allows the Dragon of Earth to blind him in his right eye -- a lesser wish Subaru had held for some time. However, in single combat against Seishirou at Rainbow Bridge, Subaru reveals his true desire: he no longer wished to kill Seishirou (having tried unsuccessfully to eradicate his feelings for the man), but instead wished to be killed by him, as thus he would exist in Seishirou's heart, even if it was only as "another victim of the cherry blossom". However, due to a spell cast by Hokuto at her death, the killing strike (a "spearhand" stab through the heart) is reversed, and Subaru kills Seishirou instead. The assassin's last words to Subaru -- which are not fully stated to the audience -- have a profound effect on the young man, being something he had not expected.
From this point, the manga and TV series differ. In the manga, following Seishirou's death, Subaru is slated to become the next Sakurazukamori, for the ritual of succession for Sakurazukamori is to kill their predecessor. Seishirou wished for Subaru to erase the mark another man had left on him. As such, Fuma gave Subaru Seishirou's eye, which he states was the only thing of his he could find at Rainbow Bridge, to replace the one the young man lost. This act completes this ritual and, after regaining his full sight, Subaru becomes the Sakurazukamori and one of the Dragons of Earth. Since he has lost the one person he wished to protect, he is rendered unable to create a kekkai any longer. His fate beyond this is unknown as the manga remains incomplete.
In the TV series, his status as Sakurazukamori is not directly alluded to. Instead, Subaru sinks back into a catatonic state, urging Kamui to look to his own wish and advising him that "it is impossible for everyone to be happy". He is, as in the manga, unable to create a kekkai, having lost the one person in the world for whom he truly cared. However, in the final episode, the rebellious Dragon of Earth dreamseer Kakyou Kuzuki allows the spirit of Hokuto to cross into Subaru's mind, to convince him to continue the fight; Subaru realizes he cares for Kamui, and joins his final battle against Fūma, protecting him from a fatal strike. Before his apparent death, Subaru urges Kamui to realize his wish. Thus, Kamui sacrifices himself to bring Fūma back to his original, kind-hearted personality and save humanity from destruction; as a side effect, Subaru is revived and lives on.
Tsubasa: Reservoir Chronicle
Subaru reappears as one of a pair of twin vampires (the other being Kamui Shirou) for whom Seishirou Sakurazuka is searching. A correlation is also shown in Subaru's relationship with Seishiro, for he seems worried about him when he asks Fuuma how he is doing in the Tokyo arc. Also, though Kamui appears to have an intense hatred for Seishiro, Subaru is much more lax about their situation of being chased and seems to silently disagree with his twin.
He makes his first appearance in the Tokyo arc, in which he talks to a comatose Sakura and urges her to wake up before she is lost to her dreams. It turns out that he himself was left in a comatose state at the bottom of a large water reservoir, leading to Kamui's fierce protection of the site, particularly from Fuuma. Eventually he wakes and is at last rejoined with his twin.
In volume one, Yūko Ichihara mentions knowing an onmyouji with a twin sister whom she has known since the onmyouji was very young. She hopes that the boy will be happy, but there are many definitions for happiness. This onmyouji is Subaru.