Kenji Sato is a young boy with short brown hair who is almost always seen wearing a baseball cap. He initially appears as a troublemaker and a staunch skeptic regarding the supernatural. Convinced that the services offered by Muhyo and Roji are a fraud, he openly dismisses their work and even destroys the sign outside their office. His disbelief is so strong that he participates in a test of courage at a local temple specifically to prove that ghosts are not real. During this event, Kenji unwittingly removes a seal and frees a dangerous blood-drinking ghost, putting himself and his friends in grave danger. The intervention and rescue by Muhyo and Roji force Kenji to confront the reality of the supernatural, fundamentally altering his worldview.

Following this life-changing event, Kenji develops a strong affinity for the pair. Despite Muhyo establishing a rule that he will not meet with clients after a case is closed unless they bring new business, Kenji persistently finds excuses to visit their office. He is eager to observe their investigations or simply spend time with them, sometimes resorting to creative methods like breaking into the office just to be there. His playful personality is reflected in the affectionate nicknames he gives the duo; he refers to Roji as Sprout-Man and Muhyo as Onion-Boy, much to their likely annoyance. While his direct involvement in major supernatural conflicts diminishes as the story progresses and the scale of threats grows beyond the city, Kenji remains a recurring presence who demonstrates loyalty and a desire to contribute. For instance, he once insisted that Muhyo and Roji take his treasured baseball cap with them on a dangerous mission, making them promise to return it personally as a sort of talisman to ensure their safe return.

Kenji has a spirited and energetic personality, always looking for excitement. His talents lie in physical activities and mischief. He is skilled in all kinds of sports and is particularly good at running away and playing pranks. He openly dislikes academic activities, sitting at a desk, playing the recorder, and eating vegetables like carrots and onions. His preferred foods include hamburgers, and he has a notable fondness for round objects, often playing with or throwing anything shaped like a ball. One of his key relationships is with a girl named Nana Takenouchi, whom he later introduces to Muhyo and Roji's office after she and her father experience supernatural disturbances, thereby expanding the bureau's circle of acquaintances.