Chosuke Nakamoto is a veteran detective in the Tokyo Metropolitan Police Department's Modern Crime Prevention Task Force. As the unit's oldest member, he transferred from the First Investigation Division. His extensive experience commands high respect from colleagues, particularly Haru Kato, who values his investigative expertise. Nakamoto chain-smokes habitually.

He employs a deliberately sympathetic and approachable demeanor during interrogations to extract confessions, yet occasionally resorts to aggressive tactics. These include threatening unrepresented suspects for information on influential entities like the Kambe group, creating friction with Daisuke Kambe, who distrusts Nakamoto's focus on his family.

Nakamoto’s reassignment to the task force directly stems from his involvement in the unsolved Sayuri Kambe murder case. After its closure due to a suspect’s suicide, he transferred alongside Yukihiro Kiyomizu. Despite Kiyomizu’s warnings about his nearing retirement and dangerous fixation, Nakamoto remains consumed by the case. He covertly pursues evidence, even requesting unauthorized reports like those on an attempted presidential assassination.

This relentless pursuit leads to his death alongside Katsuhiro Takei while investigating leads tied to the Kambe murder cover-up. Nakamoto’s killing profoundly impacts the task force: it drives Kiyomizu to fully support Kato’s hunt for their killer and contributes to Daisuke Kambe’s police resignation, as the murder exposes police corruption in concealing the Kambe family tragedy. His death underscores the personal risks of his dedication to unresolved justice.