Kenji Sato is a young boy sporting short brown hair and his signature baseball cap. Initially dismissive of ghosts, he vandalizes the sign of Muhyo and Roji's Bureau of Supernatural Investigation, convinced it's a fraud. His skepticism drives him to accept a dare to remove a shrine seal at night, accidentally unleashing a blood-drinking ghost that endangers him and his friends. Muhyo and Roji's rescue forces him to acknowledge supernatural reality.

After this event, Kenji becomes deeply attached to the duo, frequently visiting their office despite Muhyo's rule against post-case meetings without new business. He affectionately nicknames Roji "Sprout-Man" and Muhyo "Onion-Boy." His attitude shifts from disbelief to enthusiastic, sometimes intrusive, support. He actively seeks chances to observe or assist their investigations, showing loyalty and curiosity.

Kenji possesses a mischievous and energetic nature. He delights in pranks, sports, hamburgers, and playing with round objects like balls. Conversely, he dislikes studying, carrots, onions of any kind, and playing the recorder—though he surprisingly displays skill with the instrument when necessary to pacify a ghost haunting his room. He demonstrates notable athleticism, particularly in running.

During a perilous mission where Muhyo and Roji face grave risk, Kenji insists they take his prized baseball cap, making them promise to return it personally. He transforms it into a token ensuring their safe return, highlighting his emotional investment. Later, he connects Nana Takenouchi to the bureau after she and her father experience supernatural disturbances. His presence fades as major conflicts move beyond their city.

His surname "Satou" blends the characters for "help, aid" (佐) and "wisteria" (藤). His given name "Kenji" signifies "healthy, strong" (健) and "two" (二).