According to Japanese belief, Sansei are small humanoid spirits that live deep in the mountains. They range in size anywhere from about one foot tall, to three or four feet tall. Sansei’s most noticeable trait is their single leg, which is turned around backwards. They are known as the leaders of all animals which live in the mountains, and their diet mainly consists of frogs and stone crabs, of which they are particularly fond and enjoy broiling with salt.

Sansei occasionally sneak into woodcutters’ houses and mountain huts to steal salt, which they use to flavor the crabs that they eat. Though not very aggressive, they do sometimes attack humans. When this happens, if one calls out, "Hiderigami!" the sansei will flee in terror. However, if one calls out, "Sansei!" instead, that person will meet some horrible fate, such as falling ill or having their house catch on fire.

Probably for the first time, Sansei is mentioned in the Edo period in illustrated encyclopedia "Wakan Sansai Zue" (和漢三才図会, わかんさんさいずえ, literally "Sino-Japanese Encyclopedia of the three worlds") as the mountain demon from the North-Eastern Chinese province of Hebei. This book cited various chinese sources, containing conflicting descriptions of Sansey.