The word kappa itself is a combination of kawa (meaning ‘river’) and wappa (a somewhat obscure form of an archaic word meaning ‘child’). This is further complicated by many regional names for the same creature such as kawaso (which also means otter) and kawako (which is kawa plus kodomo, the modern world for child).

In any case, kappa are typically depicted as humanoid creatures roughly the size of a child that live in rivers or ponds. They usually have green, slimy (or scaly) skin, a turtle-like shell on their back, webbed feet and hands, large eyes, a bird-like beak, and a distinctive “dish” cavity on top of their heads (called sara) that holds a pool of water in the center and is surrounded by hair that often resembles plant leaves or reeds.

True to their often English-localized name of “river imp”, kappa are typically described as being quite mischievous, frequently playing tricks on humans and doing things like stealing food, especially their preferred food: cucumbers. They are also said to be competitive, sometimes challenging humans to sumo wrestling or shogi matches and sometimes perform good deeds like irrigating farmers' lands or giving people fresh fish. Once befriended, they are supposedly very loyal.

Coversely, kappa can also be malevolent with many accounts of them drowning humans and animals (particularly horses), kidnapping children, and even cases of eating or raping humans. One particularly peculiar behavior involves a mythical organ called a shirikodama that kappa are said to be able to remove from humans’ anuses to gain power and steal their soul.

When kappa are not submerged, the dish on their heads holds a small pool of water which allows them to survive on land as long as it remains wet. Since kappa are also said to be obsessed with manners, the easiest way to subdue them is to bow deeply as they will return the favor and spill out their dish, rendering them unable to move until it is replenished. According to folklore, if you do refill their dish using water from where they live, they will become your friend forever.

Like some other famous yokai such as oni and tengu, kappa have become part of pop culture while still remaining important in their religious origins. There are still Shinto shrines and ceremonies dedicated to kappa across Japan and they are also famously venerated at the Sogenji Buddhist temple in the Asakusa district of Tokyo which has enshrined the mummified arm of a kappa. Cute modern versions of kappa often appear as mascot characters for towns and businesses or on warning signs near rivers and ponds. One prominent modern example is Shiki City’s bass guitar-playing kappa mascot Kapal who won the Yuru-chara Grand Prix in 2018 as Japan’s most popular mascot.

Likewise, kappa characters have been featured in many manga and anime series, perhaps most prominently in last season’s Sarazanmai and Keiichi Hara’s film Summer Days with Coo.

The latest from revolutionary director Kunihiko Ikuhara, Sarazanmai tells the story of three boys who repeatedly transform into kappa to capture shirikodama from the butts of strange ‘kappa zombies’. Sarazanmai references nearly every aspect of kappa lore from their love of cucumbers to the aforementioned shirikodama, sumo, and even their alternative name of kawaso also meaning otter with Otter Empire as the main antagonist.

A young boy named Koichi stumbles upon a large stone that turns out to be a fossilized kappa. Naming the revived kappa Coo, Koichi and his family must adjust to life with their strange new friend, especially after the news media gets wind of Coo’s existence. Summer Days with Coo is a heartwarming adventure film exploring friendship, family, and traditional Japanese folklore.