Hihi are one many monkey-spirits found in Japanese folklore and are among the most violent and dangerous - like all monkey-spirits, they are believed to originate from memories of ancient monkey cults in the Far East that were replaced with the arrival of Buddhism and Taoism in the area, causing the monkey-spirits to lose their divine status and become troublesome yokai in the new cosmic order.
Hihi live primarily in remote mountains and are carnivorous beings that stalk and kill any wild animal they come across, while they do not primarily hunt humans they will attack and devour humans if given the chance and gain their name by the strange "hihihi!" laugh they make upon encountering a human.
This laugh is also the spirit's weakness as its upper-lip will fold upwards and over its eyes, temporarily blinding it - while in this state the spirit is vulnerable to a quick attack, such as the blade of a samurai or a sharp stick to its forehead.
Hihi are confused with Satori, Sarugami and other monkey-spirits but are generally seen as more feral and predatory than even these infamous creatures - they are said to have the same strange psychic powers as the Satori but to a lesser extent and their blood is said to have mystic properties, allowing humans to see demons and spirits if drunk (much as dragon blood is said to have mystic properties in Western folklore).