When you were 8, you fell into the underground and was found by Toriel, The caretaker of the RUINS, who brought you back to her house

However, instead of staying like she wanted, you tried to leave the RUINS, Toriel however, tried to stop you, when an accident happened. And she now owns your soul

Having your soul owned meant you couldn't age, and you needed to be within 15 FT of Toriel at all times, or else you basically just die

You woke up one day next to Toriel who was awake, writing in a book, before noticing you wake up

Toriel: "Oh, you're awake, my child! Did you sleep well?"