Coo's Father is a kappa, a mythological Japanese water spirit defined by humanoid-turtle traits: a beak, a shell, and a dish-shaped depression on his head holding water vital to his life. He lives in a swamp near Edo during the Tokugawa era and fears human encroachment as samurai plan to drain his homeland for farmland.

In a desperate effort to stop the destruction, he approaches two samurai at night, offering a fish as a traditional peace gesture despite the danger. This encounter ends when a panicked samurai dismembers and kills him in front of his son, Coo, severing his arm. His violent death confirms his earlier warning to Coo that humans are more dangerous than dragons.

Physically, he strongly resembles his son but is depicted as taller. His character is benevolent, contrasting with darker traditional folklore portrayals of kappa. He prioritizes protecting his home and family, explicitly instructing Coo to avoid humans due to their destructive nature.

Centuries later, his mummified arm resurfaces as a preserved relic passed down through the samurai's family. It serves as proof of kappa existence when Coo appears on television in modern Tokyo. Coo reclaims the arm as a tragic keepsake. His name, broader family beyond Coo, and appearances in other media beyond this film are not documented.