Most depictions show kappa as child-sized humanoids, though their bodies are often more like those of monkeys or frogs than human beings. Some descriptions say their faces are apelike, while others show them with beaked visages more like those of tortoises or with duck beaks. Pictures usually show kappa with thick shells and scaly skin that ranges in color from green to yellow or blue.

Kappa supposedly inhabit the ponds and rivers of Japan and have various features to aid them in this environment, such as webbed hands and feet. They are sometimes even said to smell like fish, and they can certainly swim like them. The expression kappa-no-kawa-nagare ("a kappa drowning in a river") conveys the idea that even experts make mistakes.

The most notable feature of the kappa, however, is the water-filled depressions atop their heads. These cavities are surrounded by scraggly hair, and this type of bobbed hair style is named okappa-atama for the creatures. The kappa derive their incredible strength from these liquid-filled holes, and anyone confronted with one may exploit this weakness by simply getting the kappa to spill the water from its head. The kappa possesses a deep sense of etiquette, so one trusted method is to appeal to this, for a kappa cannot help but return a deep bow, even if it means losing its head-water in the process. Once depleted, the kappa is seriously weakened and may even die. Other tales say that this water allows kappa to move about on land, and once emptied, the creatures are immobilized. Stubborn children are encouraged to follow the custom of bowing on the grounds that it is a defense against kappa. In addition, folklore suggests that kappa are masters of Koppo, a bone-breaking technique which they invented.