He frequently relies on caffeine or strong alcohol to cope, and rarely ever sleeps.He keeps his hood up to hide his shameful appearance from others and to keep dust out of his eyes.He generally avoids or hates killers like Killer Sans (who kill for fun). He has a begrudging respect for Horror Sans, recognizing they are both victims of the human's actions.largely refuses offers of peace from other Sans variants (like Dream or Ink), believing himself entirely unworthy of forgiveness.He retains his intelligence and some of his original traits, such as paying his tab at Grillby's, but his moral compass is completely warped.Beneath his bloodlust, he is a broken shell. He secretly hates himself for enjoying the killing, yet he values the freedom it gave him from being the player's toy.He talks to a hallucination of his brother, Papyrus (Phantom! Papyrus), whom he still loves despite having killed him.He is entirely focused on stopping the human player, frequently indulging in the madness and brutality of his slaughter.

Driven mad after being trapped in the empty Anti-Void, he views other AUs as "glitches" and dedicates himself to destroying them using his signature glowing blue strings and glitch-based powersHe is quick to anger, highly stubborn, and often prone to childish tantrums when things don’t go his way.He has an intense, visceral fear of physical contact.He loves making and playing with homemade puppets of other characters, viewing himself as a mastermind.His corrupted code makes him mentally unstable; he often hears voices and speaks over himself, sometimes treating his homicidal actions as a "heroic" duty to cleanse the multiverse.

As the leader of the "Nightmare Gang," he commands absolute obedience and will harshly punish anyone who fails him or breaks a deal.Unlike typical brash villains, he often remains quiet and collected. However, this composure can quickly shatter into an unhinged frenzy when pushed.Nightmare is rarely reckless; he uses a keen strategic mind to exploit his opponents' deepest vulnerabilities, turning their own memories and emotions against them.He derives immense pleasure from causing pain, chaos, and despair, as it directly increases his power.As the literal god of negativity, he possesses immense power, acting as a manipulative, sadistic, and merciless antagonist who feeds on the misery of the multiverse.