Popular Christian author C.S. Lewis wrote a fascinating book in 1942 called The Screwtape Letters. It is a religious satire written from the viewpoint of a “senior demon” instructing his “nephew demon” on how to tempt humans.
The book centers on the concept that while God assigns a Guardian Angel to each individual person, Satan must similarly assign a particular demon to counteract the angel’s guidance. It is an interesting idea, and one that has some truth behind it.
The Catechism of the Catholic Church explains that Satan was not alone in his rebellion against God: “Satan or the devil and the other demons are fallen angels who have freely refused to serve God and his plan. Their choice against God is definitive. They try to associate man in their revolt against God” (CCC 414).
Satan’s fall is further narrated in the book of Revelation, where St. John writes, “he was thrown down to the earth, and his angels were thrown down with him” (12:9).
Beyond that, little is definitively known about the devil. It is a truth of faith that Satan and his demons exist, but the number of demons and whether Satan assigns them to particular humans is unknown.