Kyu Renjo is the main protagonist of Tantei Gakuen Q. He is a bright, sweet, and highly optimistic boy who possesses a natural talent for detective work. Despite his sharp intellect and remarkable deductive abilities, Kyu is notably awkward in social situations, often saying the wrong thing or embarrassing himself, which sometimes frustrates those around him. In real life, he can seem immature and clumsy, but when it comes to solving mysteries, he becomes focused, logical, and exceptionally perceptive. He is considered the leader of Class Q, known for his clear and logical explanations that often surpass even his more serious classmates.

Kyu's motivation to become a detective stems from a pivotal event in his childhood. When he was six years old, he was kidnapped by criminals. He was rescued by a detective who acted as a mentor figure, whom he admired deeply and referred to as the detective man. This experience inspired Kyu to dedicate his life to becoming a detective, and he enrolls in Dan Detective School with the dream of becoming the greatest detective in the world. Unknown to him for much of the story, this mentor was actually his biological father, Satoru Renjo, who was the most trusted partner of the legendary detective Morihiko Dan. Satoru married Kyu's mother in secret to protect his family from the dangers of his profession. After Satoru was killed in the line of duty, Kyu's mother hid the truth about his father's identity and death, hoping to shield Kyu from that dangerous life. Only after Kyu was accepted into the detective school did his mother change her mind and fully support his aspirations.

Within the story, Kyu serves as the heart and de facto leader of Class Q. His good sense of judgment means he often sees the best in people even when others are suspicious; for example, he openly expressed trust and appreciation for Ryu at a time when the rest of the class distrusted him. He holds a special, affectionate dynamic with Megumi, often teasing her but also deeply caring for her. He also forms strong bonds with the other members of Class Q: Kintarou, Kazuma, and Ryu. His unwavering belief in his friends and his refusal to give up on a mystery make him a motivating force for the group. Throughout the series, Kyu grows from a boy chasing a childhood dream into a more mature detective who fully understands the legacy and risk his father carried. His development involves learning the truth about his father and coming to terms with his own destiny as a detective. His notable abilities include incredibly fast and sharp deduction skills, logical reasoning, and a knack for noticing small details that others miss, often concluding with his trademark phrase that there is only one answer.