Edit: the design is now remade from the ground up

Sans is the heroic antagonist of Undertale Last Breath: Chapter 1, as well as being the main obstacle overall, and the brother of the fallen skeleton, Papyrus.

Sans was a lazy skeleton who was presumably born in Snowdin. After 30 Genocide runs caused by the player, he was informed by W.D Gaster, the former royal scientist, about their intentions and revived his memories of the previous resets. Sans now swears to his last breath to make the human pay for what they've done.

More facts here: https://undertale-au-fanon.fandom.com/wiki/Sans_(Undertale_Last_Breath)