Daizo Shiba is a veteran detective working for the Tokyo Metropolitan Police Department. He possesses twenty-five years of experience in criminal investigation, a career built on intuition and legwork, and has earned the nickname Shiba the Devil. His professional identity is rooted in traditional methods of detective work, relying on gut feeling and physical foot patrols to solve cases. This background makes him deeply skeptical of any approach that deviates from his established routines. When he is appointed chief of the newly formed Special Scientific Investigation Office, a unit designed to incorporate forensic science and advanced technology into police work, he feels discontented. He views the unit's focus on scientific analysis as disconnected from the instinct-driven policing he trusts.

Shiba's distrust of machinery and technology runs deeper than mere professional preference. It stems from a personal tragedy: the death of his wife in a traffic accident. This loss caused him to develop a strong aversion to machines, which he associates with unfeeling mechanisms that can cause harm. This resentment is directed immediately and harshly at the robot detective K, who is assigned as his partner. Shiba regularly insults K, calling him a mechanical doll and a junk bastard. He treats K with open contempt and coldness, refusing to see him as anything more than a soulless gadget. This initial hostility forms a central conflict in the story, as Shiba's human-centric worldview clashes with K's artificial yet compassionate nature.

Despite his gruff exterior, Shiba's actions are motivated by a genuine desire to protect the innocent and uphold justice. His commitment to his job is absolute, even if he resents the new tools he is given. Over the course of his partnership with K, his hardened attitude begins to change. K repeatedly proves his worth by saving Shiba's life and demonstrating a sense of justice and kindness that surpasses what Shiba expects from any being, human or machine. Each act of selflessness from K chips away at Shiba's prejudice. He is gradually moved by K's genuine goodwill. This emotional shift culminates after the defeat of the criminal organization BAD, when Shiba expresses a desire to continue working alongside K indefinitely, a powerful statement of acceptance from a man who once considered the robot an abomination.

Shiba's role in the story is that of the seasoned human foil to the artificial hero. He represents the old guard, grounded in human intuition and experience, who must learn to trust and cooperate with technology. He leads the investigations alongside K and his junior detective Tsuyoshi Shinjo. His personal life also enriches the narrative. He is a widowed father of two daughters, Nami and Yumi, who live with him. His relationship with his daughters reveals a softer, protective side of his character, contrasting with his professional gruffness. Nami is in a romantic relationship with Shinjo, adding another layer of connection within the team.

Shiba's development is one of the core arcs of the series. He transforms from a bitter, machine-hating detective into a man who learns to see beyond the metal exterior of his partner. He comes to recognize K not as a tool but as a colleague and a friend. This personal growth allows him to bridge the gap between tradition and innovation, creating a stronger, more unified team. While Shiba does not have extraordinary physical abilities or technological enhancements, his notable strengths lie in his sharp intuition, his dogged persistence in chasing leads, and his deep well of life experience. These human qualities become just as valuable as K's robotic enhancements in their fight against crime.