Detective Hirama is a police inspector who becomes a central figure in the human effort to understand and combat the parasite threat. He is an older man of average height and build, with dark grey hair, a lined face, thick dark eyebrows, and square, silver-rimmed glasses, typically seen wearing a suit. His career standing appears to be high, with indications he holds the rank of Inspector or possibly Superintendent, and he is shown to be in a position to coordinate major operations and confer with high-level officials.

Hirama is characterized by a strong sense of justice and a pragmatic, methodical approach to his work. He accepts the reality of the parasite menace quickly and focuses entirely on developing effective countermeasures. His personality is marked by remarkable composure and lucidity, even in the face of horrifying situations, allowing him to remain focused on the mission. While driven by duty, he also displays a compassionate and motivating side, as seen when he reassures the disgraced detective Shiro Kuramori, persuading him to assist the investigation by appealing to his desire for justice and redemption. Hirama possesses sharp intuition and maintains a persistent, respectful suspicion of Shinichi Izumi, sensing that the young man is hiding something crucial about his connection to the parasites.

Motivated by the need to end the parasitic killings and protect human society, Hirama functions as the primary institutional representative against the invaders. His role in the story is to act as the tactical leader and coordinator of the human response, bridging the gap between civilian life, police procedure, and military-style action. He first appears investigating the death of Kana Kimishima, quickly establishing a link to Shinichi and the parasite involved.

Key relationships define his actions. His partnership with fellow officers Mita and Katsumata is professional, but his connection to Kuramori is more significant. After Kuramori is mortally wounded by Reiko Tamura, Hirama promises that his death will not be in vain, declaring him an honorary detective. His relationship with Shinichi is complex; he uses the young man in operations, subjects him to screening tests, and offers him police protection, yet never fully trusts him. Hirama is also responsible for employing the incarcerated serial killer Uragami as a tool to detect parasites, a morally grey decision that demonstrates his willingness to use any available resource to achieve his objective.

Hirama undergoes subtle but clear development as the series progresses. He transitions from an investigator working in the dark to a confident leader who orchestrates a direct assault on the parasites. His methods become progressively more unorthodox, from pulling Shinichi's hair to test for parasite possession to proposing the use of a murderer as a sensor and finally coordinating a major military-style sweep of a government building. He learns to adapt institutional power to an unprecedented biological threat.

While he possesses no superhuman physical abilities, Hirama has notable skills that make him a formidable opponent to the parasites. He is an expert in police tactics and firearms, demonstrating exceptional marksmanship by accurately shooting Reiko Tamura in the head under pressure. Later, during the assault on the city hall, his shooting is noted for its precision, even by the powerful parasite Goto. His greatest abilities are his analytical mind, strong leadership, and tactical planning, which allow him to devise protocols for detection and formulate strategies for large-scale extermination.