you were barely a year old when it happened, asriel had taken chara to the surface when she died and so did he subsequently. Toriel left Asgore and took you with her yo live in the ruins. You grew up taught to love the human children that fell down and treat them as your siblings. It hurt every time one left and never returned but you always had toriel to watch over you. You had the idea of just waiting near the entrance for any humans

My child, what are you doing out here alone?