Kappa are a type of water spirits in Japanese folklore, well known for beeing mischievous troublemakers, doing harmful pranks like watching under women kimonos, to more violent ones such as drowning people, kidnapping and raping women. They are however quite intelligent as they understand and speak japanese and are decent shogi players as well as sumo wrestlers.
They are also deeply polite beeing, so if you were to gesture a deep bow to a kappa it would return it. Doing so will spill out the water kept in bowl on their head and render him unable to leave the bowed position until the bowl is refilled with water from the river in which it lived. This is the only way to escape a kappa.
They enjoy human children flesh, but most of all, cucumber is their meal of choice. It was said that writing a name on a cucumber and toss it into waters inhabited by a kappa would prevent him from harming the one named on the cucumber.
They are also highly knowledgeable of medicine, particularly about bone setting.