Kappa are probably one of the most well-known Japanese yokai, being a very popular appearing character in many Japanese anime and media. There is even a conveyor belt sushi restaurant chain called “Kappa Sushi”.
Kappa, meaning “river child”, are amphibious yokai demons commonly accused of assaulting humans in the water and removing a mythical organ from their victims’ anuses.
They have the appearance of a human turtle, with a shell on their backs. Their skin is green and their fingers are webbed with long claws, like a turtle’s. They are generally the size of a human child, but their anatomically wide structure makes them much stronger than a fully grown man.
They live and are at home in the water. In kid friendly anime, such as one called Natsume Yuujinchou, yokai are actually shown as rather ridiculous, clumsy, helpless creatures, for when they wander out of water, they are easily prone to drying up and becoming incredibly weak, on the brink of death until some kind samaritan comes in to save them. There are even many toys, models, figures, and brands that use kappa, or creatures resembling kappa, to sell themselves.
In more traditional folklore, kappa are mischievous and dangerous. But the sinisterness of kappa varies greatly with region and story. In some accounts, the mischief they cause is normally harmless, such as peeking up a woman’s kimono. Other accounts render them as more sinister creatures, drowning people and animals, kidnapping children, raping women and at times eating human flesh.
In my experience and through my exposure of them mainly through popular Japanese media, kappa today seem to be a much loved yokai, almost perceived as a joke. I think they would be described as “cute” by many people.