Appearance
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 describe them with beaked visages more like those of tortoises or with duck bills. Pictures usually show kappa with thick shells and scaly skin that ranges in color from green to yellow or blue.

Kappa 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 drowning kappa) means that even experts make mistakes.

The kappa's most notable feature, however, is the water-filled depression or dish atop their heads. These cavities are surrounded by scraggly hair, and this type of bobbed hair style is named "okappa atama" after the creatures. 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. One trusted method to do this is to appeal to the kappa's deep sense of etiquette, 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.

Some tales say that this water allows the kappa to move about on land, and once emptied, the creatures are fully immobilized. Stubborn children are encouraged to follow the custom of bowing on the grounds that it is a good defense against the kappa.

Behavior
Kappa are often mischievous troublemakers. Their pranks range from the relatively harmless, such as loudly passing gas or looking up women's kimonos, to the more devious, such as stealing crops, raping women, or kidnapping children. In fact, small children comprise one of the gluttonous kappa's favorite meals, though they will eat adults as well as all manner of beasts including horses and cattle. They feed on these hapless victims by sucking out the entrails (or blood, liver, or "life force", depending on the legend) through the anus. Even today, signs warning about kappa appear by bodies of water in some Japanese towns and villages.

Kappa are said to be afraid of fire, and some villages hold fireworks festivals each year to scare the sprites away. Kappa are usually not entirely antagonistic toward mankind, however. They are curious of human civilization, and they can speak Japanese. They thus sometimes challenge those they encounter to various tests of skill, such as shogi (a chess-like game popular in Japan) or sumo wrestling. They may even befriend human beings in exchange for gifts and various offerings, especially cucumbers, the only food kappa are known to enjoy eating more than human children. Japanese parents would often write the names of their children (or themselves) on cucumbers and toss them into kappa-infested waters in order to appease the creatures and allow the family to bathe safely. There is even a cucumber-filled sushi roll named for the kappa, the "kappamaki".

Once befriended, kappa have been known to perform any number of tasks for human beings, such as helping farmers irrigate their land. They are also highly knowledgeable of medicine, and legend states that they taught the art of bone setting to mankind. Due to these benevolent aspects, some shrines are dedicated to the worship of particularly helpful kappa. Kappa may also be tricked into helping people. Their deep sense of decorum will not allow them to break an oath, for example, so if a human being can dupe a kappa into promising to help, the kappa has no choice but to follow through.

Origins
There are several theories regarding the origins of the kappa in Japanese mythology. One possibility is that they developed from an ancient Japanese practice of floating stillborn fetuses down rivers and streams. Another theory is that the kappa myth was invented as a way to explain the swollen anus commonly found in drowning victims.

The name "kappa" may be derived from the term for the robe worn by the Portuguese monks who arrived in Japan in the 16th century; they called this garment a "capa", and the monks' appearance was not unlike that of the similarly named Japanese sprites, from the loose, shell-like cloaks to their partially bald head. "Kappa" also happens to be the Japanese word for the traditional straw raincoat worn by farmers.

The Kappabashi (Kappa-bridge) area of Asakusa in Tokyo was once farmland surrounded by canals prone to flooding. In the late Edo period a raincoat dealer, Kappaya Kihachi, devoted his entire personal savings to building a new drainage system. This difficult work took longer than expected and cost a fortune. He was in despair until a kappa, whose life he had saved years before, suddenly appeared to help. With the kappa's assistance the project was completed in short order. In addition, those who had seen the Kappa were suddenly blessed with good fortune and later the Kappa Temple was built, and the kappa enshrined as a deity. The Kappabashi area is filled with images of kappa from manhole covers and street signs to store displays and statues.

Kappa in popular culture
Kappa are popular figures in Japanese anime, toys, literature, art, and even a brand of sake. Modern depictions make them much less devious, showing them instead as cute, cartoonish figures. A notable literary appearance is the famous short story Kappa by Ryunosuke Akutagawa.

They appear in a more sinister form as the organization of INKlings in Haruki Murakami's Hard-Boiled Wonderland and the End of the World. Kappa were also the inspiration for the creature in the film Ringu. A series of animated shorts on Japanese television, How to Raise a Kappa, detail the humorous trials of a young man trying to keep a kappa as a pet. Kappa appear in quite a number of video games, many of which have appeared in localized form in the West.

The "Koopas" in the Super Mario Bros. games are based on these creatures, and Super Mario World goes so far as to name a mountain with its peak filled with water "Kappa Mountain".

Western media includes notable appearances of kappa as well. The James Bond novel The Man with the Red Tattoo by Raymond Benson features a Japanese assassin nicknamed "The Kappa", because of his short height. In the Teenage Mutant Ninja Turtles series, the title characters are mistaken for Kappa during their time travels to Japan's feudal era.