Hirama is a police inspector who appears in the second half of the narrative, first introduced after the death of the character Kana. He is an older man with an average height and build, characterized by his dark grey hair, thick dark eyebrows, wrinkled face, and a pair of silver-rimmed square glasses. He is almost always seen wearing a suit, which befits his professional role.

Hirama possesses a strong sense of justice and has personally tasked himself with putting an end to the threat posed by the Parasytes. He is a pragmatic and perceptive investigator with good intuition, often sensing that the protagonist Shinichi Izumi is hiding something even without concrete proof. Beyond his professional drive, Hirama demonstrates compassion and a motivational nature towards his colleagues. He is notably one of the few detectives who genuinely sympathizes with a grieving officer named Shiro Kuramori, convincing him to assist their cause despite the personal trauma involved. His motivation stems from a duty to protect human civilization from a biological threat, representing the structured, institutional response of law and order against chaotic, predatory beings.

Hirama plays a pivotal role as the primary face of human law enforcement in the conflict against the Parasytes. He is a tactical leader who coordinates investigations and assault units, moving beyond standard police procedure to understand the enemy's structure. One of his earliest actions is interrogating Shinichi following Kana's death, a scene that establishes his suspicion of the young man. His most significant action comes when he locates the parasyte Reiko Tamura. Assessing the situation, he boldly draws his weapon, takes personal responsibility for the outcome, and shoots Tamura in the head to prove her inhuman nature to his men, which leads to a deadly exchange of gunfire. He also orchestrates the use of a serial killer named Uragami, who has a unique ability to sense Parasytes, as a tool for identifying the creatures, a morally gray decision highlighting his ruthless pragmatism. He later ropes Shinichi into the final assault on the Parasyte headquarters, valuing the young man's unique experience as an asset.

Hirama's key relationships are primarily professional. He works closely with his fellow detectives, including his partner Mita and a subordinate named Katsumata. His relationship with Shinichi is central, marked by a tense and intelligent suspicion. While he never fully trusts Shinichi, believing him to be an unstable and extremely dangerous ally of circumstance, he recognizes the boy's utility and keeps him close. He also shares a poignant relationship with Detective Kuramori, whose death he avenges by promising that it will not be in vain, declaring the fallen officer an honorary detective. His relationship with Uragami is purely instrumental, using the killer's abilities as an extra set of eyes for the investigation despite the ethical complications.

Throughout the story, Hirama demonstrates notable abilities that make him an effective adversary to the Parasytes. While not a physical powerhouse, he is a sharp marksman, displaying the skill to accurately shoot a parasyte in the head at close range. His true strengths lie in his tactical police knowledge, strategic planning, and his ability to lead special forces. He relies on methodical analysis of attack patterns and his strong leadership charisma to maintain discipline among his troops during sieges. His character remains largely consistent, defined by his unshakeable will to coordinate the human resistance and ensure that civilization does not descend into chaos. His development is less about internal change and more about his growing acceptance of Shinichi as a necessary, though distrusted, partner in the final stages of the conflict.