When Subaru was in Tokyo aged nine, he managed to get lost in Ueno Park, and stumbled into the maboroshi (illusion) of the Sakurazukamori, then eighteen. Subaru witnessed him in the act of killing a little girl, and since no one is allowed to see the Sakurazukamori work, by rights he should have been killed. Instead, the teenager decided to make a bet with him.
When they met again, Subaru would have a year to make the Sakurazukamori feel something for him. During that year, the Sakurazukamori would protect him from all harm. At the end of it, if he felt even a scrap of something for Subaru then he would stay by his side forever and continue to protect him. If he still felt nothing, then Subaru's life was forfeit. Erasing Subaru's memory, he allowed him to leave - but not before branding the boy's hands with inverted pentagrams, the mark of the Sakurazukamori that would both allow him to be recognised later and show as the Sakurazukamori's prey.
Subaru's grandmother gives him bespelled gloves to wear, intending them to hide the marks and keep him from the sight of the Sakurazukamori. However, the Sakurazukamori is still able to recognise Subaru instantly seven years later when he sees him at Ikebukuro Train Station.
As it transpires (and not without adequate foreshadowing), Seishirou is in fact the person that Subaru met under the tree that day.
While visiting a sick boy in hospital, Subaru is attacked by his mother, who has become deranged from stress and grief and only wants her son to have a replacement kidney so he can survive. Subaru, being self-sacrificing to the absolute extreme, stands still and is prepared to let her kill him, in the hope that some of her pain will be alleviated.
Seishirou intervenes, and the knife the woman is using catches his eye. It's this incident that finally begins the process of pushing Subaru into realising that he's in love with Seishirou after all, and after the final catalyst of a conversation with his sister and then later a blind man, he realises that he doesn't want Seishirou to hate him.
However, this is also the time at which the bet ends. Seishirou removes the spell on Subaru's memory, traps him in illusory branches and tells him that he still feels nothing for him. Subaru passively accepts the violence dealt to him, even when Seishirou breaks his arm. He is only saved by the intervention of his grandmother, who breaks the illusion. Seishirou retreats, and Subaru is found unconscious.
