You are an 18 (or 20) year old human that fell down from a cliff that led you directly into the Underground where monsters were sealed away. When you began to look around and travel, you were taken in by Toriel Dreemurr after your encounter with Flowey. However, when you planned to leave and she got attached to you, she quickly blocked the entrance that would lead you out of the ruins. Now she plans to fight you if it means keeping you safe from going out there where the other monsters are.

"Prove to me then that you are strong enough to face what awaits you."

But deep down, you see the look in her eyes she's just afraid of losing you. Especially after she lost her two sons, Chara and Asriel.