The younger brother of W.D. Gaster and the older brother to Papyrus, Sans was born sometime after Gaster’s split from Asgore’s Kingdom. Born and raised in Auxilium, Sans and his siblings were promptly orphaned after an incident; due to Gaster being too busy with ruling a kingdom, Sans and Papyrus were sent to live with another monster while they grew up. The monster issued the order to raise the young skeletons was the fire elemental Grillby, who took the babybones under his wing and raised them as his own. Due to this, Sans and Papyrus hold a more meaningful relationship with Grillby than they do with Gaster.

Due to Gaster’s disinterest in having children, Sans began to be groomed as the future king of Auxilium. Due to disliking his older brother’s coldness to their younger brother, Sans began to slack off and be lazy in all manners of running the kingdom, seemingly making himself unsuitable for rule. Gaster would continue to train him anyhow, and would later give Papyrus a role in the kingdom that would cause Sans to become increasingly agitated at Gaster.