Haru Katou is a detective in the Tokyo Metropolitan Police Department who serves as one of the two primary protagonists. He is introduced as an officer assigned to the Modern Crime Prevention Task Force, a unit known as a destination for police who have caused problems within the department. However, this assignment is a fall from grace for Katou, who was once a rising star in the prestigious First Investigation Division. His demotion resulted from a traumatic incident during a bank robbery case, where he was forced to fire his weapon and shot an employee who was an accomplice, an event that left him psychologically unable to use a gun.

Katou is defined by a personality that is kindhearted, empathetic, and deeply principled. He is a passionate individual with a strong sense of justice who believes that there is more to life than money and that human life is sacred. This worldview puts him in direct opposition to his wealthy new partner. He is described as an honest, straight-laced, and hardworking detective who prefers to solve problems through traditional legwork and effort rather than extravagant spending. Despite his stubbornness and occasional disgust at his partner's methods, he is a caring individual who stands up for ordinary people.

His core motivation stems from his dedication to justice and his compassionate nature. He was haunted by his past mistake and driven to continue his work, though he now approaches it with a cautious, empathetic perspective. He is motivated to solve crimes in a way that protects all lives involved, often trying to talk down suspects rather than escalating to violence, as seen during a hostage crisis on a bullet train. He is also motivated to prove that effort and integrity matter, acting as a foil to Daisuke Kambe's philosophy that money can solve every problem.

In the story, Katou's primary role is to act as the moral center and the audience's grounded perspective. He serves as the reluctant mentor and partner to Daisuke Kambe, the millionaire detective who uses his immense fortune to solve cases. The narrative follows their partnership as they clash over their opposing ideologies, with Katou representing the traditional, empathetic detective who relies on procedure and human connection. He is often shown investigating leads by asking for help from colleagues and journalists, contrasting with Kambe who simply purchases information and technology.

The most significant relationship is his partnership with Daisuke Kambe. Initially marked by conflict and disgust on Katou's part, he is repulsed by Kambe's habit of evaluating everything, including human lives, by monetary value. Their relationship evolves slowly and naturally from forced colleagues to genuine partners who learn from each other. In a moment of vulnerability, Katou invites the runaway Kambe into his modest apartment, exposing the sheltered millionaire to a normal life and deepening their bond. He also maintains connections with his former colleagues in the First Division, though these relationships are strained due to his past incident and his transfer. He is friends with a journalist named Mita, who assists him with investigations, highlighting his reliance on mutual aid rather than money.

Katou experiences considerable development throughout the series. He begins as a bitter officer, disgusted by his partner and haunted by his past trauma and demotion. As he works with Kambe, he learns that while he despises the methods, the results can save lives. He gradually stops seeing Kambe solely as an arrogant fool and begins to see him as an ally, eventually coming to his rescue in a fight. This partnership allows him to begin healing from the incident that caused his demotion, as he regains confidence by proving that his compassionate, effort-driven approach is a vital complement to Kambe's wealth. He maintains his ideals but becomes more flexible, accepting that different methods can co-exist to achieve justice.

Regarding notable abilities, Katou is a skilled detective with sharp instincts and a strong work ethic. He excels at physical surveillance and stakeouts, often spending hours gathering information that technology might miss. He is also skilled in judo, which makes him competent in hand-to-hand combat and physical arrests, compensating for his inability to use a firearm. His greatest ability is his empathy and his talent for connecting with people, allowing him to gain insights and information through genuine human interaction that his partner cannot buy.