A demon is a fallen angel. An angel can become a demon by literally falling through space. It is also possible to transform into a demon through magic.
At least 333,333 demons reside inside the circles of the planet Guiltro, ruled by the demon lord Gjlewis. These fallen angels long to be angels again and hope to achieve this by getting everyone to unsubscribe Fred Figglehorn on YouTube.
The Devil is the highest ranking demon, the ultimate personification of evil and the enemy of God and humankind. This title once belonged to Fred Figglehorn's father, but was passed onto him after he was born.