Yōkai are a class of supernatural entities and spirits in Japanese folklore. The word yōkai is composed of two kanji characters that both mean "suspicious, doubtful", and while it may be regarded as a loanword from the Chinese term yaoguai, the word yōkai has taken on multiple different meanings particular to a Japanese context.

A kappa —also known as kawatarō , komahiki , with a boss called kawatora or suiko —is a reptiloid kami with similarities to yōkai found in traditional Japanese folklore. Kappa can become harmful when they are not respected as gods. They are typically depicted as green, human-like beings with webbed hands and feet and turtle-like carapaces on their backs. A depression on its head, called its "dish" (sara), retains water, and if this is damaged or its liquid is lost, the kappa is severely weakened.