Kappa, a recurring minor youkai, resembles a humanoid duck with a green body, a beak-like mouth, and a white disc crowning his head that stores water vital to his survival. His turtle-like shell and river-dwelling nature mirror traditional Japanese folklore depictions of kappa as aquatic spirits.

Known for his kind and loyal demeanor, Kappa holds deep gratitude toward Takashi Natsume, who twice rescued him—once by replenishing the life-sustaining water on his head during severe dehydration. This bond drives Kappa to aid Natsume willingly, offering freshly caught fish and alerting him to dangers like a blood-draining exorcist targeting youkai.

In the film *Ephemeral Bond*, Kappa joins the Dog’s Circle—a group including Hinoe, Misuzu, and Chobihige—to assist when Nyanko-sensei splits into three fragments after consuming a spirit fruit. The group devises makeshift methods to reintegrate the entities, such as confinement in a box or physical collisions, though their attempts fail, allowing one fragment to escape.

Kappa remains unnamed, his identity absent from Reiko Natsume’s Book of Friends, confirming he was never bound by her contracts. Unlike folklore’s mischievous or malevolent kappa, his characterization highlights cooperation and integration into a supportive network of youkai allied with Natsume. His origins and activities beyond these documented interactions remain unexplored.