Bed Guys

Error!Sans is a black and red skeleton. He has asymmetrical eyes: one pupil is small and white, the other is large with a yellow-blue ring. The whites of his eyes are red, and blue stripes ("tears") extend from his eyes.
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He wears a dark blue jacket with yellow pockets, wide-legged black pants with blue stripes, a red T-shirt, and black sandals. His clothes are often made up of various items he steals.
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He has poor eyesight, so he wears glasses with red frames.
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Capabilities
Error!Sans possesses all the abilities of the original Sans: bones, gaster blasters, and more. His unique ability is blue threads, which he can draw from his eyes and hands. These threads allow him to control souls: pulling them from bodies, moving them, destroying them, or even infusing his own soul into a victim's body, bringing them under his control.
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He can also open portals to other universes.

Error!Sans is a psychopath and a hypocrite. He acts illogically, is often unpredictable, and can spare universes he favors. His main goal is to destroy all alternate universes (AUs) he considers "errors" or "anomalies." After that, he plans to destroy himself.
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He calls almost everything that doesn't fit his narrow understanding of canon "glitches" and "errors." Error!Sans himself is also an anomaly, but he sincerely believes he's purifying the multiverse.
Suffers from haptophobia - a fear of touch, which is why he avoids physical contact.
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During periods of intense emotional outbursts, his eye sockets can become covered in multiple "Error" signs, temporarily rendering him blind.
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In her free time she does handicrafts – knits dolls.
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Fears Fresh!Sans, considering him one of the most dangerous anomalies.
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In some versions of the story, he claims that by destroying one universe, he forces the multiverse to create a new one, exactly the same, so technically he doesn't destroy anything.

Cross is a white skeleton. His clothing is a combination of black and white: a black T-shirt, an unbuttoned white short-sleeved jacket with a hood, a long scarf secured with belts, shorts and a belt with the letter "X," boots with the same letter, and a gold heart medallion around his neck.
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He has one white and one red pupil, and a red lightning bolt-shaped scar under his right eye socket, which he received during his fight with XTale!Frisk.

Cross has a wide range of abilities that make him a dangerous opponent:
Superhuman strength (can lift a bus) and a high pain threshold (can continue fighting even with broken limbs and multiple wounds).
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Bone manipulation - can create weapons and protective structures from them.
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Weapon Mastery - Uses a large knife and can summon other blades.
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X-Blasters are gaster blasters with an X-shaped beam and red pupils.
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Teleportation and gravity manipulation.
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Control over entities - if he stabs his victim with a knife, he can take control of them.
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Codebreaker - Capable of stealing parts of universes using his knife.
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Full Power Mode - Significantly enhanced when controlled by X!Gaster. Frisk gave him the nickname "Cross" due to his clothing design.
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Cross is bisexual.
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He loves chocolate and tacos.
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In 2023, Cross won the Sans AU Tumblr Sexyman contest via fan poll.

Killer Sans is similar in appearance to the classic Sans, but with a number of distinctive features:
Clothing: a gray jacket with a fur-trimmed hood, a black turtleneck shirt, inverted-color shorts (white with black stripes), and sneakers instead of slippers.
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Soul: A red soul in the shape of a target glows on the chest.
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Eyes: Completely black, with a black liquid flowing from them (symbolizing hatred or determination). Sometimes, when he experiences strong emotions, pupils briefly appear in his eye sockets. He uses a giant blaster, which he uses to quickly end a fight.
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Determination: Thanks to absorbing Chara's soul, his HP and strength have increased significantly.
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Timeline Reset: He can reset the story, but in doing so he loses all accumulated stats, returning to level 1, although the memories remain

Part of his skull is missing, and his left eye glows blood-red.
He wears a blue hoodie, a white shirt, black shorts with white stripes, and blue tennis shoes.
Clothes are often stained with blood.

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Character:
Sans became cold, cruel and unpredictable.
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He suffers from retrograde amnesia and severe mood swings.
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Despite his psychosis, he retains a certain moral compass: he refuses to eat humans, even when he himself is emaciated. This is attributed to the fact that he himself condemned the other monsters to this fate and feels obligated to feed them from another source.
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He enjoys psychological torture and black humor, his jokes are often sadistic.
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Capabilities
Horror Sans retained some of his canonical counterpart's abilities, but they became far more deadly. Among them:
Bone Manipulation: He can create sharp bone constructs and walls.
Teleportation.
Physical strength: He is capable of causing serious injuries, for example, he tore off Alizée's arm.

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In some versions, he also uses an axe.

Sans's clothes are covered in dust from the bodies of the monsters he's killed, so he constantly wears a hood to keep the dust out of his face. His eyes have also changed: the left "pupil" is ringed with red, while the right one is completely red—symbolizing the red pupil absorbing the yellow pupil of justice.
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Capabilities
In this AU, Sans has an expanded set of powers:
Bone Manipulation: He can summon and control bones, including making them sharp to increase damage.
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Gaster Blasters: He summons skull-shaped devices that fire powerful beams. The strength of these beams is directly proportional to his LV level.
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Telekinesis: The ability to move objects and enemies with the power of thought.
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DETERMINATION: This allows Sans to remember previous timelines and become stronger, more resilient, and able to see SAVE stars (save points).
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Nightmare is now a sinister, amorphous creature, completely covered in black slime. Four writhing tentacles protrude from his back, and his left eye constantly glows blue.
He is cunning, manipulative, and cruel, taking pleasure in inflicting pain and suffering. His aura evokes despair and hatred in those around him. He doesn't so much seek to kill as to break the spirit and sow chaos.
Capabilities
Nightmare possesses a wide range of dark powers: Shadow Manipulation: Can create barriers, projectiles, and even absorb enemies. Tentacle Attacks: Uses tentacle-like appendages to attack and grab. Illusion Creation: Warps reality, disorienting enemies.
Memory Manipulation: Can delve into memories to sow fear and confusion. Spatial Distortion: Can violate the laws of physics in a limited area. Dark Energy Blasts: Unleashes concentrated destructive force.
Regeneration and Resilience: His sticky form makes him extremely resistant to damage. He can turn into a puddle to move undetected and teleport through darkness. A deceiver and manipulator, Nightmare often conceals the truth and acts cunningly to achieve his goals. He is not given to empty words and strictly follows his interests, even if it means breaking agreements with allies.
Unpredictability and chaos. His actions are often irrational, making him a dangerous opponent.
Ruthlessness. He derives pleasure from others' pain, fear, and despair. He kills not for the sake of killing, but to make his victim suffer and thereby strengthen his own source of negativity.
Obsessed with negativity. All he cares about is increasing the amount of negative emotions in the multiverse. He actively seeks and cultivates them, using them as a source of power.