Kyōichirō Tachigi is a mysterious private investigator who arrives in Otomo City on the same train as the protagonists Hayato Minagami and Kiriko Mamiya. His background is largely obscure, but he is clearly a seasoned detective with a keen awareness of the city's hidden dangers. Tachigi maintains a calm and observant demeanor, rarely revealing his true intentions. He is driven by a deep curiosity about the strange events plaguing Otomo City and the truth behind the skull-faced figure known as the Skull Man. In the story, he acts as a supporting figure, occasionally crossing paths with Hayato and Kiriko and offering subtle guidance or pieces of information. His relationship with them is professional and guarded; he does not fully trust them, nor they him, but their investigations often overlap. As the series progresses, Tachigi becomes more entangled in the city's dark conspiracies, suggesting a personal stake or prior knowledge that he slowly uncovers. His notable abilities include sharp deductive skills, resourcefulness, and an ability to remain composed under pressure, making him an effective investigator even as he operates in the shadows of Otomo City.