Isamu Kenmochi is a homicide inspector with the Tokyo Metropolitan Police and serves as a key ally to the young detective Hajime Kindaichi. A robust man with black hair and gray eyes, his physical presence is complemented by a black belt in judo, a skill he has honed since his high school days when he earned multiple titles in national competitions. Despite his imposing stature and authority, Kenmochi is often portrayed as a regular, approachable person who is more reliant on physical strength and determination than on sharp intellect.

Kenmochi's professional background is rooted in a straightforward career in law enforcement. He joined the police academy after graduating from high school in his native Nagano Prefecture, eventually rising to the rank of inspector and later chief inspector within the elite First Investigative Division. While he has reasonable investigative experience, his deductive abilities are unremarkable, and he is not naturally gifted at solving complex puzzles or riddles. This limitation is a defining aspect of his character, but it does not hinder his effectiveness, as he has learned to rely on other strengths.

Initially, Kenmochi’s personality came across as abrasive and self-centered. When he first encountered Hajime Kindaichi during the Opera House Murder Case, he was openly frustrated that the investigation was interrupting his holiday, showing little immediate concern for the victims. Over time, however, this demeanor softened considerably. He evolved into a good-natured, caring individual who demonstrates deep loyalty to his family, his colleagues, and especially to Kindaichi. He shows great concern for his wife, Kazue, and his past as a younger man, including how he met his wife while chasing off a流氓 (hooligan), reveals a history of brave, if simple, actions. In his professional life, he acts as a mentor to younger detectives and maintains an extensive network of contacts within the police force.

Kenmochi’s primary motivation is a straightforward sense of duty to solve crimes and bring criminals to justice. However, his role in the story is more specifically defined by his partnership with Hajime Kindaichi. Recognizing his own limitations in pure logic, Kenmochi places complete and unquestioning trust in Kindaichi’s extraordinary deductive skills. He frequently uses his official authority to facilitate Kindaichi’s investigations, providing the official approval the young man needs to access crime scenes and information. In return for Kindaichi’s help, Kenmochi often promises rewards, though these promises are not always kept. Their bond has grown so close that they are sometimes mistaken for relatives, with Kindaichi affectionately referring to the older inspector as "Ossan," a casual term for uncle or middle-aged man.

A key relationship in Kenmochi’s professional life is with his superior, Kengo Akechi. Despite being nearly twenty years younger, Akechi holds a higher rank, and their interactions are frequently strained by Akechi’s sarcastic and elitist attitude, which irritates Kenmochi. Despite this friction, Kenmochi privately acknowledges and respects Akechi’s formidable reasoning abilities. This complex relationship adds another layer to Kenmochi’s character, showing he can set aside personal annoyance for professional necessity.

Kenmochi’s abilities outside of judo are largely practical. While he is not a brilliant detective, he has shown moments of surprising insight, occasionally noticing key details or helping to uncover parts of a culprit’s scheme. His greatest asset is his physical courage and his ability to act as an effective bridge between the unorthodox methods of a civilian teenager and the official processes of the police. His development over the course of the series is significant, moving from a skeptical and somewhat self-serving officer to a warm, reliable father figure and an indispensable partner who embodies integrity and perseverance.