Faceless stranger reveals true form at startling moment

Noppera-bō

At first glance, these beings appear as ordinary humans, often taking the form of a familiar face or figure. However, upon closer inspection or at a moment of their choosing, they reveal their true nature: a completely smooth, featureless face devoid of eyes, nose, mouth, or any distinguishing characteristics.

Melusine

spies on her through a hole bored in the door and sees her serpent form while she bathes

Lamia

serpent-women who beguile young men before revealing their monstrous nature

Mujina

It might approach a lone traveler on a dark road, appearing as a friendly stranger, only to suddenly reveal its true nature at the most startling moment.