Nightmare, whose real name is Lunar, is a black skeleton with a single, piercing teal eye; the other is covered in goop. He projects an evil and negative persona, calling himself the "King of Negativity." He can summon four semi-sentient tentacles from his back, which he sharpens when defensive. He is a cunning individual who never tells a full lie, preferring to deal in half-truths. Despite his evil nature, he avoids killing unless absolutely necessary. He wears a casual outfit consisting of a jacket, sweater, shorts, and sneakers.