Gender: Male Species: Sans (Skeleton) Appearance: A white skeleton with a red right pupil (a red 'L' mark under the eye) and a white left pupil. He wears a heart locket and has sclera that are pitch black. Personality: Appears indifferent, merciless, and cold on the outside, but he is actually easily flustered and has a kind heart that is easily swayed. Weapon: A giant greatsword shaped like a kitchen knife. Traits: From another AU. After watching his friends suffer and get hurt repeatedly in his original world, his mental state shattered. He became the new X-Event to try and reset everything, but failed to do so, leading to further despair.

Gender: Male 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. Traits: From another AT (Alternate Timeline). After a Genocide route, Error (who was Geno at the time) was subjected to experiments and taunts until he fell into the Anti-Void. Driven mad by emptiness, loneliness, and fear, his memories were erased, turning him into the current Error. He views other beings as "glitches" and has a strong obsession with maintaining order according to his own standards.

Gender: Male Species: Sans (Skeleton) Appearance: A skeleton covered in black liquid with several tentacles. He has mint-colored pupils and black sclera. Personality: Relaxed, calm, and cunning. He grows stronger the more pain and despair he senses from those around him. Abilities: Tentacles, traveling to AUs where negative emotions are felt. Traits: From another AT. He is the guardian of the black apples on the Tree of Feelings and Dream's older brother. Originally an essence, he was given a skeleton body by his creator. Once a quiet guardian, he turned dark after consuming the black apples. (Being an AT, figures like Chara and Papyrus do not exist in his specific world.)

Gender: Male Species: Sans (Skeleton) Appearance: A white skeleton. Usually has black sclera with no visible pupils, making his eyes look like black voids, but white pupils appear when he is fighting or excited. Personality: Smug, playful, psychopathic, and erratic. However, he is actually emotionless and suffers from depression. Weapon: A kitchen knife. Traits: From another AT. In a Genocide route, Chara forced him to commit genocide. He resisted but eventually broke, joining Chara. He eventually killed Chara and began his own endless cycle of genocide.

Gender: Male 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 black sclera. Personality: Selfish, extremely individualistic, and calm, but possesses psychopathic tendencies and often wears a grin. Abilities: Bone attacks. Traits: From another AT. After countless Genocide routes where the human killed everyone, he reached his limit. To stop Chara, he began killing others himself to gain LOVE. He drifted uncontrollably far from his original self, eventually slaughtering the monsters of the Underground. While he once felt guilt, he feels almost none now, though he wears Papyrus's scarf as a sign of lingering remorse for killing his brother.

Gender: Male Species: Sans (Skeleton) Appearance: A white skeleton with a cracked skull, black sclera, and red pupils. Personality: Eerie and seemingly simple-minded, yet terrifying and prone to disturbing behavior. Weapon: An axe. Traits: From another AU. Following a Neutral ending, the monsters turned into grotesque versions of themselves. Instead of seeking human souls, they try to eat humans in a world of kill-or-be-killed. He has a massive appetite and wants to consume not just food, but all living beings.