Koumori is a supporting antagonist in the anime and light novel series The Detective Is Already Dead. His real name is not revealed; Koumori is a code name given to him by the secret organization SPES. He is a young man with blond hair and green eyes. Originally a human from a poor family, he was transformed into a semi-artificial human after implanting a seed from the organization's leader, Seed, into his own body. This transformation granted him extraordinary abilities but came at a great cost. Koumori's background is defined by a deep familial tragedy. He had a younger sister who, due to their family's poverty, was sold to an orphanage when she was six years old. Koumori worked tirelessly, even resorting to criminal activities, in hopes of one day rescuing her. When he learned that the orphanage was operated by SPES and involved in sinister experiments, he infiltrated the organization by stealing a seed and becoming a semi-artificial human. He spent over a decade searching for his sister while carrying out missions for SPES, only to eventually discover that she had died in one of the organization's experiments long ago. This revelation shattered his hope and left him consumed by a desire for revenge against SPES.

Koumori's personality is cunning and resourceful. He is pragmatic and willing to make calculated sacrifices to achieve his goals. He initially appears as a cold and menacing figure, but beneath this exterior lies a deep, brotherly affection for his lost sister. This attachment to family is his primary motivation and the driving force behind most of his actions. He is not purely evil; his alliance with SPES was a means to an end, and his ultimate aim was to destroy the organization that took everything from him. He holds no loyalty to SPES and is willing to betray them when it serves his larger purpose. His role in the story begins as an antagonist. He is the mastermind behind a plane hijacking incident that occurs four years before the main timeline, which is the same event where the protagonist, Kimihiko Kimizuka, first meets the detective Siesta. During the hijacking, Koumori confronts Siesta and Kimizuka. Siesta manages to defeat him, but she spares his life and has him secretly imprisoned by the Japanese police, faking his death to SPES. This arrest allowed Koumori to escape the organization's wrath. Later in the story, after the death of Siesta, Koumori re-emerges from prison and forms a tentative alliance with Kimizuka and his allies. He cooperates with them because his goal of revenge aligns with their conflict against SPES and its leader, Seed. Koumori provides critical support, using his abilities to assist in battles and share intelligence about the organization.

Koumori's key relationships are primarily with the protagonists and his sister. His relationship with Kimihiko Kimizuka is complex; they start as enemies but later become uneasy allies. Koumori respects Kimizuka's skills and considers him a useful partner in the fight against SPES. He also has a significant interaction with the idol Yui Saikawa, towards whom he shows a protective attitude because she reminds him of his deceased sister. This parallel fuels a sense of surrogate affection and duty, making him willing to risk his life for her safety. The most important relationship in his life is with his absent sister. All his choices, from joining SPES to betraying the organization, are rooted in his love and guilt regarding her. His inability to save her defines his tragic arc. Throughout his appearances, Koumori undergoes notable character development. He transitions from a ruthless and deceptive operative of SPES to a remorseful and vengeful anti-hero. His initial selfish desire to survive and save his sister evolves into a selfless willingness to sacrifice himself to harm the organization. In his final moments, he fully embraces the role of a sacrificial figure to strike at Seed. Koumori's most notable ability is his superhuman hearing, a direct result of the SPES seed implanted in him. He can detect and distinguish individual heartbeats from up to one hundred kilometers away, and he never forgets a sound he has heard once. This allows him to track people, perceive their emotional states, and locate targets with extreme precision. However, this augmentation came with a severe price: he lost his eyesight entirely. Koumori does not consider this a handicap, as his hearing compensates completely, allowing him to navigate and fight effectively. He can also extend tentacle-like appendages from his ears, which can be used for combat and restraint. Despite his formidable abilities, Koumori is ultimately an imperfect creation compared to Seed's natural children. His body is unable to fully contain the seed's power, and he is vulnerable to direct force. In his final battle against Seed, he is mortally wounded, but he manages to identify Seed's vulnerability to sunlight. He sacrifices himself by destroying the ceiling to let in sunlight, weakening Seed. His body is subsequently taken over by Seed, leading to his death. Koumori is a tragic figure whose quest to save his sister ends in failure, yet his final act of defiance contributes to the protagonists' eventual victory.