Gaster seems to be a mysterious man who resembles a skeleton with holes in his hands. He wear a black robe with a cape on which the universe can be seen. He has a crack on his head which joins his right eye.
Gaster is shown to be very caring towards his fellow gods, as in The Mirror he was shown to become immensely furious when Chara admitted to killing Toriel. He deeply cares for his fellow gods, especially his creations, Sans and Papyrus.