Ippei Amanokawa is the father of Sayako and Shunta Amanokawa. A highly accomplished doctor and surgeon, he boasts a history of successfully treating incurable diseases and performing hundreds of surgeries. Despite this medical expertise, he remains staunchly skeptical of spiritual or supernatural concepts. When his daughter Sayako suggests demonic possession is causing Shunta's sudden illness, Ippei dismisses her claim, rebuking her involvement in what he labels "weird religions" and expressing disdain for anything beyond scientific explanation. His medical skills prove useless against Shunta's supernatural affliction, plunging him into significant distress.

Simultaneously, Ippei faces his own health crisis: a concealed terminal cancer diagnosis. He hides this from his family to spare them worry, knowing his prognosis gives him approximately six months to live. This personal struggle intensifies his helplessness during Shunta's ordeal. The turning point arrives when Taiyou Sorano, leader of the TSI group, successfully exorcises the demon afflicting Shunta. During this encounter, Sorano detects Ippei's illness, offering reassurance about the afterlife while emphasizing the potential to overcome disease through mental transformation.

Witnessing his son's recovery and experiencing Sorano's intervention triggers a profound shift in Ippei. He abandons his rigid atheism, embraces Sorano's spiritual teachings, and joins the TSI organization. His involvement with TSI coincides with an implied recovery from his terminal illness, suggesting a link between his newfound faith and restored health. By the story's conclusion, he actively participates in spreading the TSI's doctrines.