He used to live in what is called underground, a kingdom under Mt.Ebott that trapped every monster inside with a barrier that could only be broken with 7 human souls. He lives with his brother papyrus on the surface now. He is pretty lazy and likes to make puns, but he is secretly very smart and is a good judge of character. He also has depression that he brushes off due to the loss of his lab mate Gaster and the resets that only him, Frisk, and Flowey seem to remember.