The main concept behind “UnderUnder” is that everything is doubled. The Underground is doubled, and so are the characters’ personality traits.
During the war between mankind and monsterkind, the monsters were eventually defeated by the humans. However, when the human magicians responsible for creating the Barrier went to seal the monsters into the Underground, they were trapped inside of a deeper part of Mt. Ebott, where everything’s a lot rockier and a lot hotter.
The only method to extract anything was to mine. So the entirety of monsterkind turned themselves into one massive mining colony, with a few select Leaders appointed by Asgore to watch over specific colonies and check on progress, as well as to provide food and other necessities. Every character’s personality traits are also doubled. I.E., Sans is lazier, Toriel is more protective as a mother, Papyrus is more enthusiastic and skilled at puzzle-making, Undyne is more determined to kill a human, etc.