Keisuke Yamao is a 34-year-old unemployed man and the primary antagonist from the fifteenth film in the Detective Conan series. His background is rooted in a deep-seated gambling addiction that led him down a path of severe criminal activity. To settle his debts, he committed a robbery and murder at a jewelry store, stealing a significant amount of diamonds. During his escape back to his hometown of Kitanosawa while driving drunk and without a license, he accidentally hit and killed Natsuki Toono with his car. While disposing of her body, a young boy named Touma Tachihara witnessed the scene. Yamao attempted to kidnap the child, resulting in Touma falling from a cliff and entering a comatose state for eight years. Yamao then hid the stolen jewels in Kitanosawa before turning himself in to the police for the hit-and-run, for which he served an eight-year prison sentence.

Upon his release, Yamao returns to Kitanosawa, only to find that the area where he hid the diamonds is now submerged under a newly constructed dam. His sole motivation becomes the recovery of the jewels, a goal he pursues with ruthless determination. When his friend, Shogo Hikawa, discovers a newspaper article detailing Yamao’s past crimes, Yamao murders him to prevent the truth from coming to light. Believing he is running out of time, he escalates his plan to a massive scale, aiming to bomb the dam. This act would not only allow him to search the sunken area for his loot but would also destroy the dam, potentially flooding the downstream village and causing many innocent casualties. His motivations are purely selfish, driven by greed and a complete disregard for human life.

In the story, Yamao serves as the culprit whose past sins connect all the major characters. He is the one who killed Natsuki, the sister of Mizuki Toono, and caused Touma’s long coma. His recent release from prison and his return to town set the plot in motion. He is directly responsible for the attempted bombing of the dam and the subsequent danger faced by Conan and the other characters. Key relationships define his antagonistic role. He is a former childhood friend of Mizuki Toono, Fuyumi Tachihara (Touma’s mother), and Shogo Hikawa, but he betrays these bonds through murder and violence. His actions directly victimized Touma Tachihara, whose recovered memories become crucial to solving the case. He directly confronts Conan Edogawa at the climax, taking him hostage and brutally assaulting him atop the dam while intending to kill him.

Keisuke Yamao exhibits a personality marked by extreme selfishness, vanity, and a complete lack of remorse. He is portrayed as a coward who would sacrifice anyone, including his friends and an entire village, to achieve his goal of retrieving the diamonds to pay his gambling debts. He is also sadistic, as shown when he taunts Conan about his failure to save his friends before attempting to murder him. Throughout the film, he shows no development or redemption; he remains fixed on his greedy objective, becoming more violent and desperate as his plans are threatened. He does not possess any unique investigative or intellectual abilities. His notable abilities are more practical and destructive, including the use of firearms, the construction and detonation of bombs, and a willingness to use physical violence. He successfully orchestrates a complex scheme involving blowing up a tunnel, derailing a subway, and destroying cell phone towers to buy himself time. In the end, despite all his efforts, he is apprehended by the police and is expected to face severe punishment for his extensive list of crimes, which include multiple murders, attempted murder, terrorism, and robbery.