Kapaeru is a demon appearing as a hybrid of a kappa and a water sprite. His name combines the Japanese words for "kappa" and frog ("kaeru"), reflecting his amphibious nature, and he frequently experiences anger when mistaken for a frog. As part of the Yokai Patrol, his responsibilities include gathering intelligence on escaped demons. He utilizes abilities like teleportation, wall-walking, body stretching, swimming, toxin resistance, and spraying water from his scalp. He operates undercover in the human world for extended periods, sometimes deceiving humans into believing his form is merely a disguise.

In narrative contexts, Kapaeru often serves as comic relief, providing assistance during battles despite lacking extensive offensive capabilities. Specific plot developments include his mutation after exposure to a demon's cauldron and a body-swap incident where his consciousness occupies another character's form. He maintains a close relationship with a human contact who supplies case-related information, holding this individual in high regard. However, in certain storylines, his loyalty becomes ambiguous, leading to acts of betrayal against teammates Enma and Yukiko. Tragic outcomes occur in one continuity where both Kapaeru and his human contact are killed by a demon possessing Yukiko.

His role within the team dynamics involves both cooperation and friction. He works alongside Enma and Yukiko in confronting escaped demons, yet his actions occasionally create obstacles for the group, including withholding critical information or actively undermining their missions. Kapaeru's long-term residence in the human realm positions him as a key source of local knowledge for the Yokai Patrol's operations.