Sans

Killer looks like the original, but his jacket is gray-blue with a fur hood, a black turtleneck, in front of which his variable soul is always visible (see below), and white shorts with a black stripe. Instead of slippers, he wears gray-blue sneakers. His face changes depending on the state of his soul. Killer has a clouded mind, wanting to kill or bring pain to others, because it gives him pleasure. His soul is defective - it is something like a target on the side of the heart (if he even has one). This is the result of a perfect balance of the monster soul and the human soul (the inverted and normal hearts compensated each other). He has 4 states: first - the soul looks like a monster soul, but red and with a border, Color can switch it to this mode; second - the soul slowly rotates in the shape of a target; The third state—the soul becomes a solid, turbid spot, spinning at an incredible speed (at this point, it no longer pulses, as if the heart had stopped); Nightmare is able to switch his soul into this mode. The fourth state is a kind of "berserker mode," in which Killer is completely uncontrollable, and the hatred from his eyes becomes almost solid.[20] Interestingly, although Killer's soul was merged with Chara's, there is no Chara mind in Killer, only memories of her.

Initially, even before the bullying, Nightmare was quite a daring and very self-confident brother. He always wanted to learn something new, even if it was dangerous or not very useful for a variety of purposes. This habit has carried over to the current Nightmare, as he reads books from time to time.

Nightmare is quite attracted to negative emotions and will do anything to obtain as many as possible. Killing victims is pointless, as without them, the negative emotions he craves won't be there. However, he does kill those who break their pacts with him, or force them to remember their past selves before their corruption, or when they anger him (though in these cases, he will still try to make his victims suffer and experience the deepest negative emotions, such as doom, despair, or other similar feelings).

Nightmare never lies, he only conceals the truth. He takes deals quite seriously, which is why he kills those who break them without question.

Gray-blue jacket with fur in the hood. A black turtleneck, in front of which the Killer's changing soul is always visible. Shorts are white with black stripes. Instead of slippers, I wore grey-blue sneakers. The face changes depending on the state of the soul. Killer has 4 states: First, the soul looks like a monster's soul, but it's red and has a border. Second, the soul slowly rotates in the shape of a target. Third, the soul becomes a solid, turbid spot, rotating at an enormous speed (at this point it no longer pulsates, as if the heart had stopped). The fourth is the "berserker mode," in which Killer is completely uncontrollable, and the hatred in his eyes becomes almost dormant. Important: Although Killer's soul was merged with Chara's, he does not have Chara's mind, only her memories.