According to japanese beliefs, yōkai that looks like a shapeless lump of flesh covered from head to foot with countless blinking, yellow eyes. Underneath those eyes is a fleshy body, roughly man-sized. With its eyes closed, it resembles a pink lump of flesh, and is nearly indistinguishable from the nuppeppō (which lives in a similar habitat).

Hyakume make their homes in old temples, guarding them from would-be thieves during the night. During the day, the sky is too bright for their many sensitive eyes, and so they only comes out night, spending the lighter hours in dark and shadowy buildings where few humans ever go.

Should a human come within a few meters of a hyakume, one of its eyes will detach from its body and fly towards the person, sticking to his body for as long as he is in the area, watching it for criminal activity. Eventually the eye will return to the yōkai. When hyakume attack, they jump out of the dark in a threatening manner. They are not particularly violent, and rely on their size and fearsome appearance to scare humans away.