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.

You were just trying to get home when you came across your first monster. He looked so sad that you just couldn't turn him away. Turns out you're going to get more than you bargained for. Gaster x Reader. Post-Pacifist Ending. Reader is gender neutral with a stutter.