A bright, sweet and optimistic boy who is also a precocious detective. In real life, keeps saying the wrong things and embarrassing himself. Otherwise, he is considered to be the team's leader and is known for his logical explanations of cases. He has a good sense of judgment regarding to people (when everyone distrusted Ryu, he expressed his open appreciation for him) but often says things that infuriate Megumi.
Kyuu had been a single child of a single parent (his mother) for as long as he could remember. He didn't have a father, but there's a certain man who had been acting as his mentor when he was little. When he was six years old, he was kidnapped and held as a hostage by some criminals, but his mentor rescued him. In reality he was none other than Kyu's father (Satoru Renjo, Morihiko Dan's very best partner). Realizing the high risk of being a detective, Satoru formerly married Kyu's mother in secret to keep his new family safe. After Renjou's death, Kyu's mother hid the truth from Kyu for years, trying to shield him from the danger that the life of a detective is always in; after he was accepted in the DDS, Mrs. Renjou changed her mind and supported him fully.