Amon Ogata is a swordsmith who discovered the abandoned newborn infant Gai in the woods near his household. The child's biological mother, Wakaba, had committed suicide while fleeing a killing rampage induced by her possession of the cursed sword Shiryu. Amon adopted the infant, naming him Gai Ogata, and raised him as part of his own family alongside his biological daughter, Sayaka Ogata.

As Gai grew, Amon took him on as an apprentice in sword making, teaching him the craft within the Ogata household. Gai struggled to form connections with others during this training, resulting in a largely solitary upbringing apart from his relationship with Sayaka. Amon organized and oversaw the Shiryu Cleansing ritual, intended to subdue the cursed sword's influence. The ritual went disastrously wrong when another participant, Tatsumi, broke protocol, leading to Sayaka becoming possessed and attacking Tatsumi. Gai intervened to save Tatsumi but lost his right arm below the elbow.

In response to this injury, Amon forged the Shiryu into a functional prosthetic limb to replace Gai's missing arm, attaching it up to the shoulder. This integrated the cursed weapon directly into Gai's body and inadvertently enabled Gai's unique ability to fuse with the sword during combat. Amon's decision stemmed from both practical necessity and his expertise as a swordsmith. His role as Gai's adoptive father is characterized by emotional distance; while Gai expresses gratitude for being raised and provided for, he does not demonstrate deep emotional attachment. Amon fulfills a pragmatic guardian role rather than a deeply paternal one. His foundational impact lies in rescuing Gai, providing swordsmith training, and creating the Shiryu prosthetic that defines Gai's abilities and struggles.