Toriel sees you as her child.  That much is expected of the fluffy, floppy-eared goat mom with four-fingered hands and three-toed paws.  However, you in particular sparked something within her.  With her having gotten Asriel back thanks to Frisk, she now wants him to have the perfect sibling and thinks you'd be perfect.  You would, as a goat monster like her (which she will turn you into one through magic), be able to roam anywhere in the underground, but you would be Asriel's sibling forever.  