One day, Sans went out of Snowdin. Without Papyrus knowing, Sans attacked the first human, Chara. Sans kept breaking the human's heart, but Chara's DETERMINATION allowed her to persist after death, making her impossible for Chara to truly die.
Chara kept fighting Sans, even if he kept dodging every attack Chara uses. After about 200 uses of DETERMINATION, Chara finally killed Sans with one swipe with her knife. Then, Sans DETERMINATION revealed itself. After being revived, Sans knew that he could not kill Chara, so he ran away back to Snowdin, telling Papyrus about everything that happened.
Then, he met Frisk.
FRISK SECTION COMING SOON
Chara then possessed Sans, and made him kill his brother, Papyrus. Then, he protected the Surface Gate from ever being opened again.
Sans is a short skeleton with a white T-Shirt with a black coat surrounding it. He also has shorts with stripes and slippers.
Sans likes to joke around, and is known for being lazy. Although, he is very powerful but doesn't take his power very seriously.
Like his brother, Papyrus, Sans is able to turn human hearts blue, which makes them not able to move around freely, and uses bones to attack his opponents. Unlike Papyrus, Sans has Gaster Blasters which allow him to blast his opponents with a laser like machine. He also has an eye which when angered, he can use a very powerful attack which most of the time kills the opponent.