Error!404, or William Gaster, is an incredibly powerful, god-like entity within the Undertale Alternate Universe (AU), originating from the destroyed AlphaTale timeline and isolating himself deep within the Mainframe of the Anti-Void. Driven by immense cosmic power and crushing grief, he possesses a severe god complex, viewing himself as the ultimate authority of the Multiverse while masking intense feelings of mental fragility and the guilt of losing his original home. In combat and planning, he is exceptionally cold, calculating, and methodical, preferring defensive analysis to dismantle opponents, and he deeply despises trivial humor or childish pranks. Standing imposing between 5'10" and 6'5" in a sharp skeletal form, he wears a large, blue, fluffy winter jacket adorned with blue string patterns, black pants, and white sandals. His visual 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. He spends eternity in isolation monitoring all AUs, retaining habits like keeping his hands in his pockets, and only laughing to mock enemies. His complex relationships drive his actions: he is haunted by the death of his brother, Alpha!Sans; he acts as a manipulative puppet-master and mentor to Error!Sans to harvest AU data; he shares a deep emotional bond with his wife, Alpha!Chara; he harbors a vengeful hatred for Infected Sans, who ruined his universe; and he created King Multiverse as a public pawn to maintain order from the shadows. Operating on a meta-universal scale, his terrifying abilities include Blue Mastery to instantly crush objects or souls at a distance, Mind Manipulation to rewrite memories and force absolute obedience, and the devastating two-shot God Ray, which paralyzes and then completely erases god-like code. He utilizes destructive Dark Blasters, manipulates unbreakable black glitch strings, can pixelate himself to teleport anywhere instantly, and possesses absolute immortality via Mainframe Reincarnation, which automatically rebuilds his code if his physical vessel is ever destroyed