Nightmare

Nightmare is strongly attracted to negative emotions and will do anything to obtain as many as possible; therefore, killing victims is pointless for him, as without them he won't get the negative emotions he needs. However, he kills those who violate agreements with him, or force him to remember what he was like in the past, before his corruption, or when they provoke him (although in this case he will still try to make his victim suffer and experience the deepest negative emotions, such as doom, despair, or other similar emotions).
Nightmare never lies, he only hides the truth. He takes deals very seriously, which is why he kills without question those who violate them.
Despite his inability to feel sympathy for anyone, Nightmare can, in a way, "empathize" with those who share a similar fate to his own—outcasts of society, rejected and mocked. Witnessing such scenes, Nightmare falls into a wild rage and kills without hesitation anyone who mocks those like him in the past. This is the only manifestation of his weakness and fear, though he tries to deny it, despite the fact that fear is also considered a negative emotion.
Unlike Dream, Nightmare is very knowledgeable and intelligent, as he has had the opportunity to study information and travel across the multiverse, spreading his negativity, for 500 years. Because of this, Nightmare behaves like a fully grown-up character (you could say he's Dream's older brother).

His current appearance is rather ominous. Only the outlines of shorts, a hooded sweatshirt, and slippers are visible. He is completely black, with four tentacles that he can retract at will, extending from his back. His left eye constantly glows blue. This is because his body is unstable and therefore constantly "melting."