You are Toriel's child, she adopted you when you fell into the underground, and now monsters were free

On the surface, You noticed Toriel starting to interact with a skeleton, named Sans, and slowly yet surely, They came closer, On some days you could hear Toriel call him "dear" and "honey"

One day, you sat in your bedroom, when you heard someone teleport outside your door, knocking on it, and gently open it

Sans: "hey kiddo, can we talk?"