Kyuu Renjou is the main protagonist of the series and a student of Dan Detective School, where he is placed in Class Q, the elite group designated as the qualified successors to the school's founder. He is a mild-mannered and highly intelligent young detective with remarkably fast deduction skills. In everyday social situations, he is awkward and often says the wrong thing, frequently embarrassing himself and sometimes infuriating his classmate Megumi Minami with his blunt or tactless remarks. Despite this social clumsiness, he is fundamentally bright, sweet, and optimistic, with a cheerful disposition and a boyish immaturity that contrasts sharply with his focused, serious demeanor when solving mysteries.

His primary motivation stems from a formative childhood event. When he was six years old, he was kidnapped by criminals and held hostage. He was rescued by a detective he referred to as the Detective Uncle, who inspired him to dedicate his life to becoming a great detective and to enroll in Dan Detective School in pursuit of that dream. Unknown to Kyuu for most of his childhood, this mentor was actually his biological father, Satoru Renjou, who had been Morihiko Dan's best partner. Because of the high risks associated with being a detective, Satoru married Kyuu's mother in secret to protect his new family. After Satoru died, Kyuu's mother hid the truth about his father from him for years, trying to shield him from the dangers of a detective's life. She only revealed the truth and fully supported his ambitions after he was accepted into Dan Detective School.

Within Class Q, Kyuu is regarded as the team's natural leader. He is known for his logical explanations of cases and his ability to synthesize information and arrive at conclusions quickly, often providing the crucial deductive leaps that solve the most complex mysteries. He possesses a strong sense of judgment about people; when the rest of the class distrusted Ryu Amakusa, Kyuu openly expressed his appreciation for him and trusted him. His role in the story is that of the central problem-solver and the moral compass of the group, often providing the intuitive and logical synthesis that ties together the team's various specialties. He works closely with his teammates, who each possess unique abilities: Megumi has a photographic memory, Kintaro Toyama has physical strength and street instincts, Kazuma Narusawa is a computer hacking prodigy, and Ryu Amakusa is a cool-headed genius with a mysterious background. Kyuu harbors affection for Megumi, often teasing her while also deeply caring for her protection.

As the story progresses, Kyuu develops from a naive but gifted boy into a more confident and capable detective. He is deeply committed to the ideals his father taught him, encapsulated in his motto that a detective must never give up or the case will remain unsolved. He is also driven by the legacy of his father and the desire to live up to the example set by the detective who saved him. In addition to his deductive reasoning, he is a keen observer who notices small details that others overlook, and he is highly analytical and strategic in his approach to crime-solving, often relying on his own intuition and intelligence to work through even the most difficult cases.