Sans Gaster

"Relax, kiddos! I'm just tugging your tails!"

-Sans joking with the girls, The Great Unknown.

Sans is a skeleton monster that lives in the Underground with his brother, Papyrus. He is friends with Frisk and Flowey and the Royal Judge of the Monster Council.

Appearance
Sans is a short, paunchy skeleton with a round skull, a wide toothy grin, and large eyesockets with white socket pupils within them that disappear when he's serious or angry. He wears a blue jacket, black fingerless gloves, a white shirt, black shorts with white stripes, and dark blue sneakers. However, in the original Undertale, he is shown and described as wearing salmon-colored slippers.

Personality
Sans has a mostly laid-back and calm personality, though he can be serious at times (for example, when he's telling the girls that history better not repeat itself with him protecting them the same way he protected Frisk). He is also quiet at times, and teases people whenever he gets the chance.

Chapter 11: Snowdin
Sans and Papyrus meet the girls with Frisk and Flowey outside of the Ruins. They chat a bit before heading through Papyrus's ice puzzles for Snowdin, meeting a few monsters along the way.

Chapter 12: Waterfall
Sans and Papyrus go through Waterfall with the girls, Frisk and Flowey to Undyne's house.

Trivia

 * It's heavily suspected that Sans has a crush on Frisk.
 * In the World of Fantasia series, Sans and Papyrus have a living mother named Arial.
 * Sans is older than Papyrus, despite the difference in height.
 * Sans is able to open and close his mouth at will--unlike his original counterpart, who doesn't seem to open his mouth.