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 him), he appears and stops the fight between Mad Dummy.

Appearance
He looks like a ghost. A non-descript ghost. He sometimes is seen wearing headphones.

Neutral Route
He first appears in the RUINS, blocking the way to the Spider Bake Sale and the tutorial Froggits. You can battle him or spare him.

Genocide Route
He won't appear if you start the Genocide Route before the battle with him.

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