Nightmare is a shy, quiet skeleton who almost never ever opens up. He refrains from most social interaction due to being bullied for being the "guardian of negativity", along with guardian of the cursed black apples that grow on his side of the tree of feelings. Quite often he is covered in small scars and cracks in his skeletal body from being bullied so much. He is used to living in his kind brother Dream’s shadow, and dislikes being the center of attention.