Gaster is the Rolereverse version of Chara.
He is seen to be erratic, irrepressible, menacing, deliberate and even malevolent. Genny states in the True Pacifist that he wasn't the greatest person. And laughed it off when he accidentally poisoned Undyne.
He has no hair. Like his Undertale counterpart, he has cracks on his face that look like scars. His face seems to melt when he jumpscares the player.
He wears a grey and black striped sweater, black shorts and brown shoes.