As there have only been two demons thus far in the chronology of the series, it is difficult to define rules for the species as a whole. Shachath, an angel, recognizes them to be a related species, referring to the demon she was speaking to as "a dark cousin" and "fallen". This may imply that demons were once angels that have fallen from grace, thereby turning their backs on humanity and God - a clear homage to the events of the Bible, in which Lucifer gathered rebellious angels to turn against Heaven. This can be supported in the fact that the demon within Sister Mary Eunice danced mockingly in front of a crucifix of Jesus Christ, mouthing the words of "You Don't Own Me" by Lesley Gore - "don't tell me what to do, and don't tell me what to say!", all while screaming and crying in rage.
Some demons, such as the one which possessed Mary Eunice, only appear to reside within the minds of humans whose bodies they have corrupted, passing from one mortal victim to another at the moment of their host's death, while other demons, such as the Addiction Demon, appear to exist indefinitely as a tangible personification of a fatal flaw in humanity. In both cases, it appears that demons can only thrive via the feeding on or torture of mankind.
Demons have displayed a variety of supernatural powers - most prevalently, the ability to either read the minds of humans or retain an inherent consciousness of humans' past deeds and regrets, which they use to torture their victims and drive them to the brink of insanity, death, or sinfulness. The demon possessing Sister Mary Eunice has displayed a number of unnatural abilities consistent with powers that witches also possess, among them telekinesis, concilium, clairvoyance and transmutation.