Kosuke Mizutani is a central figure in the mystery of Detective Conan: The Raven Chaser, initially presented as the prime suspect in a series of murders before his full backstory is uncovered. He was the boyfriend of Nanako Honjou, a young woman who died two years earlier in a hotel fire in Kyoto. The couple shared a deep bond, frequently spending time together stargazing, a pastime that heavily defines his character and motivations.

Grief-stricken and unable to move on from the tragedy, Mizutani was manipulated by Nanako's brother, Kazuki Honjou. Kazuki convinced him that one of the seven survivors of the elevator incident had maliciously pushed Nanako out, leading to her death. In truth, Nanako had voluntarily stepped out of the overcrowded elevator to allow the others to escape. Driven by this misdirected sense of vengeance and profound sorrow, Mizutani allows himself to be set up as the scapegoat for a series of killings orchestrated by Kazuki. Mizutani's personality is defined by quiet desperation and a willingness to sacrifice himself for a cause he believes honors Nanako. He is not a violent criminal but a broken man whose judgment is clouded by loss.

His background in astronomy serves as his primary notable ability and is critical to the story. Conan Edogawa deduces that the locations of the murders align with the Ursa Major constellation, a pattern directly linked to Mizutani and Nanako's hobby. This realization allows Conan and the police to predict the final crime scene, the Tokyo Tower. In his role within the narrative, Mizutani acts as the emotional core of the serial murder subplot. At the Tower's pinnacle, Conan confronts him as he prepares to confess to the crimes and commit suicide by poison. Conan reveals the truth about Nanako's selfless sacrifice and exposes Kazuki as the true killer. Mizutani's development in this moment is profound; he realizes that Nanako would not have wanted him to stain her memory with murder or his own death. He abandons his suicide plan and chooses to live, freed from the lies that consumed him. His key relationships are with the deceased Nanako, whose memory dictates his actions, and with Kazuki Honjou, the antagonist who exploits his grief. His interaction with Conan directly leads to the resolution of the case and his own redemption.