Orphaned young under unknown circumstances, Koutarou Amon was raised in a Catholic orphanage by priest Donato Porpora. This peace shattered when he discovered Donato was a ghoul butchering the children. Donato spared Amon's life, an event defining his future. Rescued by the CCG during Donato's capture at age twelve, Amon joined their Ghoul Investigator Training Academy. He graduated top of his class and partnered with Senior Investigator Kureo Mado. On their first case involving the ghoul "Applehead," Amon's reluctance to suspect an elderly woman nearly killed him; Mado intervened, teaching him to trust a partner's instincts. This partnership cemented Amon's dedication to eradicating ghouls, driven by preventing children from suffering his fate. Amon possessed a strong sense of justice, viewing ghoul extermination as necessary to mend a "wrong" world. His motivations stemmed from his orphan trauma and the shattering of his foster father's image. He trained rigorously, developing an athletic, muscular physique, and typically wore a black suit with a white overcoat and a silver cross necklace. He wielded quinques like Doujima 1/2 (a koukaku club/lance) and Kura (a koukaku quarterstaff), the latter inherited from Mado's will. Key early investigations included pursuing Ryouko and Hinami Fueguchi, where he desecrated a grave for evidence, justifying it as moral for justice. His first confrontation with the half-ghoul Ken Kaneki ended with Kaneki sparing his life after destroying Doujima 1/2. This mercy sparked Amon's curiosity about Kaneki and began challenging his black-and-white worldview. Mado's death at Touka Kirishima's hands intensified his guilt and obsession with capturing "Rabbit" (Touka) and understanding Kaneki ("Eyepatch"). Promoted to First Class, Amon partnered with Mado's daughter, Akira Mado. Their relationship was initially strained by her resentment over her father's death, but gradually deepened through shared grief and duty. He also mentored the admiring Seidou Takizawa. During the Raid on Kanou's Lab, Amon witnessed Kaneki's kakuja rampage and encountered the Yasuhisa twins—former CCG trainees turned ghouls—further complicating his perceptions. Investigating Donato yielded cryptic advice to seek "Alice" over the "White Rabbit." On the eve of the Owl Suppression Operation, he rejected Akira's romantic advance, prioritizing their professional roles. Leading Squad 4 in the Owl Suppression Operation, Amon confronted Kaneki at a blockade. Using Arata Proto II armor and upgraded Doujima, he initially overpowered Kaneki but was critically wounded, losing his arm. As he lay incapacitated, Tatara and Aogiri attacked his squad. Takizawa sacrificed himself to save Amon, but both were captured. Amon endured experiments by Akihiro Kanou, who implanted him with Yoshimura's kakuhou, transforming him into an artificial one-eyed ghoul. Declared dead and posthumously promoted to Special Class, he resurfaced during the Rushima Landing Operation with a kakugan in his right eye and a kagune replacing his right arm, which regenerated with varying finger counts after battles. He adopted a hooded cloak to conceal his identity. Becoming a ghoul caused a profound identity crisis. Amon grappled with emptiness, viewing his CCG career as a waste and struggling to find purpose. His worldview shifted as he experienced ghoul suffering firsthand, recognizing they exhibited human emotions like benevolence and hatred. He could no longer sustain his former hatred, realizing his crusade was fueled by manipulated grief over Donato. This introspection led him to reconcile with his foster father during the JSDF assault, acknowledging his affection before delivering the killing blow. He sought redemption by saving former comrades, notably confronting a deranged Takizawa to restore his sanity. Despite initial shame, he reconnected with Akira, saving her from Takizawa and caring for her cat during her recovery. Encouraged by Touka Kirishima—whom he forgave for Mado's death—he confessed his feelings to Akira, sharing a kiss. Amon's ghoul abilities included an ukaku kagune for projectile attacks with high accuracy, a grotesque kakuja form with multiple eyes and blade-like protrusions, superhuman strength, agility, durability, and regeneration. He adapted Doujima by integrating his kagune to reform the weapon. He allied with Kaneki after understanding his goal of coexistence, assisting in the fight against V and Furuta. His journey culminated in aiding the effort to create peace between humans and ghouls.