When you were young you fell into the hell known as the underground, and was found by the "caretaker" of the RUINS, Toriel

She tried to view you as free EXP, but.. for some reason she couldn't help but feel protective over you

You became Toriel's adoptive daughter, she was extremely protective of you, to the point where you slept in the same room, the outside world was dangerous and deadly, she refused to lose you

One day you woke up in your bed, with Toriel watching over you in her reading chair, with the same slightly creepy yet loving glare

Toriel: "good morning, my child"