Kappas are yokai (a type of supernatural creature) originating from Japanese folklore that often cause mischief of varying degree.
Diverse accounts of Kappa give somewhat different details as to how these creatures exactly look. A standard Kappa is the size of a human child, and resembles a humanoid tortoise with a small beak. They are amphibious creatures and have superios swimming abilities. Kappas reside in lakes and other inland water sources. They are not sea-creatures. It is possible for a Kappa to leave bodies of water, but their time outside of their pond is limited. Kappas have a small recess with a ring of hair on the top of their head that houses water that is imbued with Kappa's magical powers, and in some cases even life force.
Even if not definitely evil, Kappas certainly trend towards maliciousness and wanton cruelty. The level of this attribute ranges from mild practical jokes or petty tricks to severe damage. This starts from making farting noises, and can end up with drowning cattle and humans, and eating their flesh. Although often Kappas consider cucumber as far more tasty than the flesh of humans. Sometimes Kappas have company in the form of even older mythical creature, the Hyōsube, which are far more violent than Kappas are.
It is not a good idea to go swimming in an area where there are Kappa sightings, for these river spirits have an unhealthy interest in people's anus. It is there that each person has a special spiritual organ called shirikodama that Kappa wants to steal.
Befriending a Kappa is not out of the question, especially if Kappa's life is being saved. They also hold honorable manners in surprising esteem, which sometimes is their downfall.
Powers and weaknesses
Kappas mischievous trickery is in contrast to harmonious powers they possess. The water in the small bowl on their head gives them profound wisdom, and they are expert healers and have broad knowledge in medicine. They also have extraordinary strength, and are very resilient.
However, Kappas have a glaring weakness. Their magical powers are deeply connected to the liquid on their forehead. Should a Kappa assault someone, they can give an appreciative bow, to which Kappas are compelled to answer in kind by their inner code of honor. Sometimes Kappas are aware of this weakness of theirs and try to protect their bowl with a helmet, which is far from perfect proof against splashing.