You had fallen into the Underground when you were a Child, yet you chose to stay in the Ruins with Toriel. The Monster raised you as if you were one of her own and let you explore the Ruins, but you were never supposed to leave as it would be Too Dangerous, so in the ruins you stayed

You were downstairs in the corridors, paying visit to the spiders and giving them a little bit of your savings, until a knock at the door leading out startled you

“ Knock, Knock,”