DEMONS - are supernatural beings who occasionally come to the human world to form Faustian contracts with humans, offering a range of services in return for the future consumption of the individual's soul.
Although they seem to exist for the sole purpose of consuming souls of humans. It's indicated that they don't need a Faustian contract to consume souls however forming one allows the demon to groom the soul into being a dish worth devouring.
There are two aesthetics of demons: one, their masters' orders are absolute and two, obey those orders while keeping their contracts. These two aesthetics are the core of most demons' actions. Although they seem devoid of the most basic principles of love, loyalty and faith they seem to highly value the aesthetic value of the contract that they make.
Those that are contracted receive a mark of their contract somewhere on both the demon's and the human's bodies, the location of the mark depends on where the one forming the contract wants it, for example, as Sebastian's contract is on his left hand, while Ciel's is on his right eye. The contract is comparable to a scar that allows the demon to be able to locate his or her master at any time. For the human, it allows him or her great authoritative power over the demon, but all the same he or she can never escape the demon.
In order to create the contract, a human has to summon a demon. However, it seems that they do not need to be aware of the existence of demons, attempt to summon a specific demon, or even summon one intentionally.
While a demon can consume human food it lacks any deseriable flavor or value to them. The only thing that has a delcious taste to them is that of a human soul. They can not consume another demon, Angel or Shinigami in such a way.
Abilities
Demons are shown to have a variety of abilities, including:
• Enhanced speed.
• Enchanced Strength
• Enhanced endurance: strong durability;
• Quick Healing ( Wound that would never heal/ aka a missing arm leg will repair itself in a few days)
• Enhanced sense of smell.
•Enhanced sense of hearing.•Transformation: form-switch into animals or either copy the appearence and voice of other humans;
•Soul staining: It is not known precisely how this works or what it entails, but Grell states that a demon's hands and lips have the ability to stain a soul.
•Elements control: such as fire, water, ground, wind, plants.
• Manipulation: mind control or "Sweet-talk" into gathering information or persuading to do onto the demon's liking, not effective against shinigami.
•Body Manipulation: can manipulate the body of someone to act according to their wish.