Hanpei Hattori is a private detective who becomes a key human ally to the android protagonist Jiro in the anime series. Financially struggling and without any active cases, he is hired by Mitsuko Komyoji to locate Jiro, who has run away following a misunderstanding with her. Initially unaware that Jiro is an android, Hanpei believes he is searching for Mitsuko's missing older brother. This beginning establishes his entry point into a conflict far larger than a simple missing persons case.

In terms of personality, Hanpei blends comedic traits with a deep-seated sense of loyalty. He is frequently fearful and easily frightened by the dangerous situations and monstrous robots he encounters, embodying what could be described as a cowardly archetype. Despite his fears, he consistently rises to the occasion, showing tenacity and a willingness to put himself in harm's way to help his friends. He claims to be a descendant of the legendary Iga ninja Hattori Hanzo, a boast that adds a layer of eccentricity to his character. His motivations evolve over the course of the story. While he is initially driven by the need for money and a desire to help a worried sister, his experiences with Jiro and the DARK organization open his eyes to a greater sense of justice, transforming him into a truly reliable supporter.

Hanpei's role in the story is primarily that of a human anchor in a narrative dominated by androids and super-science. He acts as an investigator, tracking down clues and locations for the Komyoji family. He also serves as a mediator, attempting to use logic and emotional appeals about family bonds to reach Jiro when others cannot. Beyond investigation and mediation, Hanpei takes direct action, such as climbing a radio tower in a failed attempt to rescue Jiro from the monster Carmine Spider, an act that leads to him witnessing Jiro's transformation into Kikaider for the first time. In the climax of the series, he plays a critical role by securing Dr. Komyoji's brain and ensuring its safe return, an act that demonstrates his ultimate trustworthiness. He is also noted as the only ordinary human to have directly confronted the series' main antagonist, Professor Gill.

His key relationships center on the Komyoji family and Jiro. He forms a strong bond with Jiro, whom he respectfully calls "Mister Jiro," and becomes a persistent and loyal friend. He works closely with Mitsuko and her younger brother Masaru, and he has an apprentice named Etsuko Sarutobi who assists him in his detective work. His relationship with Jiro deepens from a simple client-and-subject dynamic to one of mutual respect and reliance, with Jiro entrusting him with vital tasks.

Hanpei undergoes notable development throughout the narrative. Early in the series, he is portrayed as somewhat greedy and even betrays the group for self-preservation or money. However, as the story progresses, he sheds these mercenary qualities and becomes an invaluable and selfless ally, willingly acting as a decoy in dangerous plans. His abilities also seem to grow alongside his character. While he is initially shown to be clumsy, such as playing the guitar poorly, by the final episodes he can perform tasks with remarkable skill, even flawlessly imitating Jiro's guitar playing. He claims to hold black belts in judo and karate, and as a self-proclaimed ninja descendant, he demonstrates techniques like the "Nawanuke no Jutsu" (rope escape technique). His most practical skill, however, is his uncanny ability to track down Jiro across various locations in Japan, a talent that proves essential to the plot time and again.