What is Demon:
Demon is a word that comes from the Greek daimon, which means "genius" or "spirit" not personified. This genius or spirit can also be understood as a force, but never as a person with a will or with a character.
Due to the fact that the Gospels tell different stories about exorcisms practiced by Jesus, the belief has spread that demons are always negative or devilish forces, or they are the devil himself. However, these are not to be confused with the personification of the devil.
Demons, geniuses, or spirits can be good or bad. In the realm of spiritual beliefs, it is believed that demons, as a non-personalized force, act through people in the form of possession, and therefore can propel them to both creation and destruction, both to good as to evil. Now, in both cases, the "demon" must be released so that the possessed person can once again make use of his will and freedom.
In the time of Jesus, epilepsy was considered a demon, since it supplanted the will and conscience of the subject. In the same way, many diseases that generated this effect in people were had by demonic forces.
This type of force must be distinguished from the concept of the devil. The word devil always alludes to that or that which divides, separates or slanders. Many times he is personified through the figures of Satan or Lucifer, "adversary" of God and man. On the other hand, a demon, genie or spirit does not have its own entity, plans or will. Therefore, it does not constitute a character.