Gaatarō is a kappa from Kappa Country who accompanies the human protagonist Sanpei Kawahara back to the human world. This pairing stems from Sanpei's initial journey to Kappa Country, where Gaatarō receives the assignment to study human society.

This kappa character bears different names across adaptations: Kanpei, Kanko, and Gaatarō. The name Gaatarō specifically identifies the character in the animated film version. While usually portrayed as male, one live-action adaptation featured the character as female.

Within the film, Gaatarō exhibits a markedly kind and open-minded nature relative to other kappa. This disposition fosters a deepening brotherly bond with Sanpei as their story unfolds. Their relationship strengthens through shared adventures, with Gaatarō progressively learning human customs alongside Sanpei.

The core premise involves Gaatarō residing in the human world under the strict condition of concealing his true kappa identity. This necessity leads to scenarios where Gaatarō temporarily assumes Sanpei's place in human society. Despite a physical resemblance to Sanpei, Gaatarō possesses distinct personality traits and occasionally displays greater competence than Sanpei in certain human activities.