Sans (Evil)

Sans also known as the first Sans or the fallen Sans, is the first Sans to fall into the Underground. Sans is also the fallen Sans that the player names at the start of the game, and not the controllable character who is played throughout the entirety of Nyendertale.]

Sans looks strikingly similar to the protagonist, down to a "similar fashion sense," as said by Papyrus, and just like the protagonist, has an ambiguous gender. Both appear to have been around the same age after falling into the Underground. Sans's SOUL is also the same color as the protagonist's, which is indicated by the red SOUL on Sans's coffin