Unlike every other NPC, they have no collision physics, and approaching them will cause them to react in surprise and vanish while a sound effect plays. They are black and white with a distorted, cracked face.
In Room 269 (room_mysteryman), a figure widely believed to be Gaster can be found. Gaster does not have a known official form, but there are several rooms and unused assets found in Waterfall that point to what he may have looked like.
