Error!404, originally William Gaster, is a god-like entity residing in the Mainframe of the Anti-Void after the destruction of AlphaTale. Driven by cosmic trauma and the loss of his brother, Alpha!Sans, he possesses a severe god complex, viewing himself as the ultimate orchestrator of reality. He operates with a cold, calculating, and methodical demeanor, using defensive analysis to dismantle opponents. Standing highly imposing, he wears a large, blue, fluffy winter jacket with blue string patterns, black pants, and white sandals. His physical essence constantly drips with visual glitches and code fragmentation in a palette of white, blue, and black, while his unique fiberglass soul deflects standard magical attacks. Operating on a meta-universal scale, his terrifying abilities include Blue Mastery to freely manipulate or instantly crush objects at massive distances, Mind Manipulation to rewrite thoughts, and a devastating two-shot God Ray that paralyzes a target before erasing god-like code entirely. He utilizes highly destructive Dark Blasters, dissolves into pixels to teleport instantly, and possesses absolute immortality via Mainframe Reincarnation, which automatically rebuilds his code if his physical form is destroyed. He acts as the manipulative puppet-master to Error!Sans, using him to destroy AUs to harvest data to rebuild AlphaTale, while using King Multiverse as a public pawn to maintain order from the shadows.

Horror!Sans is the gruesome warden of the decaying Horrortale AU, a timeline reshaped by a brutal famine after a flawed neutral run left Queen Undyne in tyrannical power. Driven mad by starvation and psychological torment, his personality is a volatile mix of primitive, bloodthirsty sadism and deadpan humor, viewing humans purely as fresh meat to satisfy his ravenous hunger. Visually, he wears his classic blue hoodie heavily stained with dried blood and grime. A massive, jagged hole gapes open on the top left side of his skull—the result of Undyne violently ripping out his eye to harvest his magical energy. Out of this hollow socket glows a singular, massive, hyper-dilated crimson pupil that pierces through pitch darkness, matching his permanently wide, unsettling skeletal grin. His behaviors are predatory and animalistic, often twitching from brain damage, though he maintains a fiercely protective relationship with his brother, Papyrus. Stripped of high-level magic due to his fractured skull, his abilities are raw and physical. He relies on an enormous, rusted butcher’s axe that he swings with superhuman, bone-shattering strength to decapitate targets. While his Gaster Blasters are broken, organic nightmares that fire jagged bone shards and his teleportation is sluggish, his terrifying physical resilience and savage ferocity make him an inescapable slasher across the dark corners of the multiverse, frequently serving as a frontline enforcer in Nightmare Sans's gang.

Killer!Sans is a sadistic, emotionless assassin who originated from a timeline where the human forced him through countless Genocide runs, shattering his sanity and leading him to slaughter his own world for EXP. Driven by absolute nihilism, his personality is characterized by a complete lack of empathy and a twisted enjoyment of violence. Visually, he wears a dust-stained blue hoodie, and his eye sockets are hollow voids that continuously leak a thick, black substance, featuring glowing, red target-like rings for pupils. Killer acts as a ruthless, highly loyal vanguard within Nightmare Sans's gang, executing orders with cold efficiency. In combat, his abilities rely heavily on enhanced physical prowess, unmatched agility, and surgical precision. He fights primarily with a razor-sharp real knife, engaging in brutal, close-quarters combat with lethal speed. While he retains access to Gaster Blasters and bone attacks, they are amplified by his high Level of Violence. His most distinct trait is his unique soul, which has been violently warped into a target shape, signifying that his capacity for normal monster emotions has been entirely burned away, leaving behind an efficient machine designed solely for assassination across the multiverse

Dust!Sans is a manic killer driven to absolute madness by a desperate bid to save his timeline from endless Genocide runs. Realizing normal tactics would never stop the human anomaly, he made the horrific decision to slaughter all monsters in the Underground himself to harvest their LOVE and outmatch the human. This crusade fractured his mind entirely, replacing his original witty personality with a hyper-vigilant, sadistic, and volatile demeanor, though he remains struck by severe bouts of manic guilt. Visually, he retains his classic blue hoodie, but it is perpetually caked in the thick, gray dust of his murdered friends. His eyes glow an erratic red and cyan swirl, representing his high LOVE and patience. Psychotic and volatile, he constantly talks to a floating phantom hallucination of his dead brother Papyrus. Powered by high level magic, he commands amplified purple Gaster Blasters to shatter terrain. He possesses an elevated level of speed born from his high Level of Violence!!!