Napstablook's design is that of a simple ghost: mostly black and white, a long white body, two eyes with black sclerae (the white parts of the eyes on normal eyes), white pupils, and a mouth. A thin black line separates the eyes. After encountering Napstablook in Waterfall, they wear a pair of generic black headphones for the rest of the game.

Napstablook is a dour introvert with low self-esteem. Napstablook cries in battle, which may indicate some form of depression, anxiety or sorrow. Despite this, Napstablook is courteous to others and treats the protagonist as a guest when they visit Napstablook's home (even if attacked in the Neutral Route). They often employ self-deprecation and apologize for any inconveniences to be polite.

Napstablook blocks the protagonist's way in the Ruins and pretends to be asleep by repeatedly saying "Z" out loud. If the protagonist cheers them up, Napstablook thanks the protagonist for being so kind and then leaves. If fought, Napstablook admits that they were lowering their HP to be polite before leaving.