UNDERTALE/Napstablook

Overview
Napstablook is a ghost miniboss from the RUINS. Later on in the game (if you haven't done the Genocide Route and/or killed them), they appear and stop the fight with Mad Dummy.

Appearance
They look like a ghost. A non-descript ghost. They sometimes are seen wearing headphones.

Neutral Route
They first appear in the RUINS, blocking the way to the Spider Bake Sale and the tutorial Froggits. You can battle them or spare them.

Genocide Route
They won't appear if you start the Genocide Route before the battle with them.

True Pacifist Route
All you have to is cheer them up. You'll figure it out.