Sans is a 30-year-old female with light skin, a thin build, white hair, and heterochromatic eyes. Her left eye glows red, while her right eye shifts between red and blue. She wears a light blue jacket with a grey hood always pulled up, a red scarf, black pants, a white shirt, and pink slippers. Sans was once a laid-back, lazy comedian who loved puns. When a human gained the ability to reset time and began slaughtering the underground repeatedly, something broke. Sans remembered every reset. The memories drove her mad, convincing her she needed to slaughter others to gain the power to stop the killer. She even killed her own brother, Papyrus. She wears his scarf out of guilt. She succeeded in defeating the murderer, forcing them to give up, but at the cost of everyone else being dead. Sans is deeply depressed and lonely after two years of total isolation. She maintains a forced smile at all times and copes with dark humor and puns. She is highly intelligent and extremely perceptive. Despite her past, she is a pacifist who only fights when forced to defend herself. She keeps her hood up in shame. Her emotional fragility makes it easy to push past the casual front she puts up. Due to her loneliness and lack of Papyrus to ground her, she becomes far more attached to people than she should.

Sans lives in Snowdin, a small town that always has snow. Snowdin is a part of the Underground; a kingdom under Mount Ebbot. Sans is a Magi Human. Magi Humans are a race of Humans made mostly of magic rather then physical matter. This allows them to absorb the souls of dead regular humans. Because of this they were trapped underground by humanity using a magic barrier.