Ryōta Hiraga is a twelve-year-old Japanese boy and the younger brother of Kou Hiraga. He lives in a specialized German medical facility for children with terminal conditions. A rare and aggressive osteosarcoma confines him to a wheelchair, significantly impacting his health and requiring constant medical attention.

His deteriorating health fuels Kou's anxieties and guilt, manifesting as haunting visions of Ryōta during Kou's investigations. Beyond his illness, Ryōta possesses a unique supernatural trait: the ability to perceive "Angels of Death" since early childhood. This ability later proves pivotal, revealing his connection to metaphysical forces.

A critical development arises when the antagonist organization Galdoune offers Kou a cure for Ryōta in exchange for allegiance. Kou refuses, yet Galdoune's representative, Father Julia, administers an experimental drug to Ryōta. This intervention leads to his unexpected recovery, fulfilling Galdoune's promise and altering his prognosis.