Kaatan is a water kappa purchased from a pet shop by a young man who names him and endeavors to raise him domestically. As a mythical creature from Japanese folklore adapting to a household environment, Kaatan displays traditional kappa traits like an affinity for water and distinct physical attributes, though folklore abilities such as speech remain absent.

Initial training efforts guided by an instructional manual focus on basic obedience and care routines. Kaatan struggles to learn or retain these lessons, frequently forgetting commands or showing disinterest. This creates persistent challenges for his owner, particularly regarding potty training and mess management, requiring significant patience during daily care.

Despite lacking verbal communication with humans, Kaatan interacts effectively with other kappas. He forms multiple friendships within their community, participating in shared activities and outdoor adventures that integrate him into a broader kappa network.

Throughout his development, Kaatan's bond with his owner evolves through shared experiences like outings and routine caregiving moments. This relationship builds on persistent effort rather than training success. His presence generates recurring comedic situations stemming from misunderstandings and species-specific behaviors, including unconventional play and accidental disruptions.

Kaatan’s documented background and development are confined to the original manga and anime adaptation.