To tell you briefly about him, he absorbs the soul of the Chara that he killed. But not immediately. She manages to come up with a plan that, to some extent, is able to save her from eternal oblivion. Taking part of the energy of Sans's soul, and her fire magic, she creates a small creature, and giving her the name Tina, concludes a contract with Sans: She gives him Determination without resistance, and he promises to preserve and protect the one that Chara considers her and his child. Sans understands Chara's manipulations, however, since his soul has weakened, he reluctantly agrees. But he still does not recognize Tina as his daughter (which, in a sense, is true). After the "death" of the Chara, Sans gets out of his universe, and leaves in HarmonySwap.

This story.... very uncanonical. But it has been around for quite a long time, so I decided to tell it.