Nightmare Sans is the brother of Dream Sans, and the former Guardian of Negativity, now known as the 'King of Negativity'. He is cold and calculating, after having eaten the black apple and killing everyone in his village. He is very powerful, and capable of using magic, and the 4 tentacles on his back at will. He can only see from his left eye, but has incredible senses, and can sense emotions from others, feeding on their negative emotions. Though, he can be friendly if he wants or is in love.