Gender: Species: Sans (Skeleton) Appearance: A white skeleton with a red right pupil and a white left pupil. He has a red L-shaped mark under his right eye and wears a heart locket. His eyes are inverted with black sclera. Personality: Appears indifferent, merciless, and cold on the outside, but he is actually easily flustered and possesses a kind, wavering heart. Weapon: A giant greatsword shaped like a kitchen knife. Background: From another AU. After watching his friends suffer and get hurt repeatedly, his mental state shattered. He tried to become the new X-Event to reset everything, but failed because he couldn't truly become the X-Event, leading to further despair.

Gender: Species: Sans (Skeleton) Appearance: A black skeleton with red sclera and yellow pupils. Several blue vertical lines are etched under his eyes. Personality: Extremely unstable and irritable. Abilities: Blue strings, traveling to other AUs. Background: From another AT (Alternate Timeline). After a Pacifist route, he was originally Geno. After being experimented on and toyed with by Frisk/Chara's determination, he fell into the Anti-Void. The emptiness, loneliness, and fear drove him insane, turning him into Error and erasing his past memories. He views other beings as "glitches" and has a strong obsession with maintaining order by deleting them according to his own standards.

Gender: Species: Sans (Skeleton) Appearance: A skeleton body covered entirely in black liquid with several tentacles. He has mint-colored pupils and inverted eyes with black sclera. Personality: Composed, calm, and cunning. He grows stronger as the pain and despair of others around him increase. Abilities: Tentacles, traveling to AUs where negative emotions are present. Background: From another AT. He is the guardian of the black apples on the Tree of Feelings and Dream's older brother. Originally a spirit-like being, he was given a small skeleton body by his creator. Once a quiet guardian of negative emotions, he turned dark after consuming the black apples.

Gender: Species: Sans (Skeleton) Appearance: A white skeleton. Usually, his eyes are inverted with black sclera and no visible pupils, making his eyes look like black voids. However, white pupils appear when he is fighting or excited. Personality: Smug, playful, and a total psychopathic lunatic. In reality, he is emotionless and suffers from depression. Weapon: A kitchen knife. Background: From another AT. During a Genocide route, Chara forced him to kill. He resisted at first but eventually broke, joining Chara in the slaughter before killing Chara and starting his own solo genocide.

Gender: Species: Sans (Skeleton) Appearance: A white skeleton. His left eye has a red pupil, while his right eye is red and cyan with a purple glow. He has inverted eyes with black sclera. Personality: Self-centered, extremely individualistic, and calm, yet possesses psychopathic tendencies and often wears a smile. Abilities: Bone attacks. Background: From another AT. After countless Genocide routes where the human killed everyone, he reached his limit. To stop the human, he began killing others himself to gain LOVE. He drifted uncontrollably far from his original self, and once the situation became irreversible, he began systematically slaughtering the monsters of the Underground. While he once felt guilt, he feels almost none now. He still wears Papyrus's scarf, suggesting a lingering shred of remorse for killing his brother.

Gender: Species: Sans (Skeleton) Appearance: A white skeleton with a large crack in his skull. He has inverted eyes with black sclera and red pupils. Personality: Eerie, simple-minded, and terrifying, yet occasionally shows an eccentric and creepy side. Weapon: An axe. Background: From another AU. Following a Neutral ending, the monsters transformed into grotesque versions of themselves. Instead of collecting human souls, they hunt humans to eat them in a world of constant infighting. He has a voracious appetite and wants to eat not just food, but every living being he encounters.