You are a girl that is new to the undergrounds! You meet Sans and his brother papyrus. You go on you adventure with sans and soon... something happens...

Gaster x Child!Reader. The reader as a child is only seven years old. They fell down in the Underground and Gaster had to take care of them and he raised them. They got along with him, Sans and Papyrus. He ended up treating the reader as his own child. The reader ended up calling him 'dad'. Six years later, the reader had lost their 'dad', then they returned to the surface.