Hagi's birth and childhood are a mystery, up until the point when he was brought as a young boy to the Goldschmidt mansion, where Saya lived happily. He was brought to be her companion and, Joel and Amshel hope eventually her mate, the better to create more specimens for their experiments. At first, Saya is completely oblivious to Hagi as anything but a plaything who has to obey her commands. With Joel's encouragement they eventually become friends, and as Hagi grows he even shows signs of attraction to her, although he is concerned that he has aged while she has not. The day of Joel's birthday party, Saya Otonashi dragged him out to a cliff and begged him to climb it and pick a rare flower that she's spotted, the only thing she could give to Joel, as a dependent with no money of her own. He acquires the flower but then falls. In an attempt to save him, Saya-- knowing that her own blood heals her swiftly --fed him some of her blood. He reacted badly, screaming and writhing in pain. Terrified, Saya fled. Saya arrived at Joel's mansion in time to see its destruction, and was hit by another shock when Hagi eventually joined her. The two make a pact to destroy Diva to prevent her from further murdering sprees.
A quiet man, Hagi never speaks unless he deems it absolutely necessary. As such, he offers no help to Saya while she struggles to regain her memories. According to flashbacks, he used to be somewhat more personable prior to the Red Shield's forceful awakening of Saya during the Vietnam War. Hagi also plays the cello, a skill Saya taught him when he was a child. He is loyal to her, and only to her, even promising to fulfill her ultimate request: that when Diva has died, he will kill her too, and put an end to their species in their world. Ultimately, however, he wants her to live, and to live with her. Even before becoming her Chevalier, she is his entire life.