Rachel is one of the protagonists, and the author's persona. She has the title of Esheye Telari and will be able to obtain and use every power in existence. Her partner/guardian is Soul, who is represented by a tail with a glove-like hand.
Rachel seemed like a ordinary human until one day, she had a negative encounter on Earth and ran away. After a mishap, she was shot and did die before her Will activated enough to call out to her MotherOch (short for Ochena, and is female, sorry), who heard her and transported her away from Earth (to Natura) and unlocked the first part of her powers.
Upon meeting the monsters, Rachel revealed her knowledge of them immediately. While she didn't outright reveal her love for Sans immediately, it became clear to the others how she felt and Mettaton was the one to suggest it to Sans while Rachel was incapacitated.
After Rachel not only saved Papyrus, but countless other monsters who had been taken by the Human Empire, Sans' feelings started to change. After a while, his feelings became mutual when she showed that she not only understood him, but cared about him immensely.