Satan's primary mission is to lead us away from God, by any means necessary.
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.
Biblically speaking it may be the case that multiple demons are actively trying to lead us away from God. In the Gospel of Mark, Jesus performs an exorcism of a possessed man and discovers a “legion” of demons (see Mark 5:9). A Roman legion at the time of Jesus was between 3,000 and 5,000 soldiers. This means that at the very least 3,000 demons were tormenting a single individual!
Jesus also cast out seven demons from Mary Magdalene in an episode that is briefly mentioned in Scripture (see Mark 16:9).
This methodology seems to correspond with the nature of evil, which has a tendency to act like a pack of wolves surrounding its prey.
The good news is that God is more powerful than any legion of demons. For example, Jesus mentioned the vast number of good angels at his command when he said, “Do you think that I cannot appeal to my Father, and he will at once send me more than twelve legions of angels?” (Matthew 26:53) Jesus is saying that the Father can send down as many as 60,000 angels to help him! We always have our Guardian Angel at our side, but God can always send down more angels to us if we call out to him in our time of need.
So even if a particular demon were “assigned” to tempt and torment us, we have no cause to fear demons when we are living in close union with God. As St. Paul wrote in his letter to the Romans, “If God is for us, who can be against us?” (8:31)