Demon (悪魔; Akuma, also translatable as Devil) is a supernatural being who occasionally comes to the human world to form a Faustian contract with a human, offering a range of services in return for the future consumption of the individual's soul.
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 reason for most demons' actions. There is no faith or loyalty that prompts them to act otherwise. However, demons are highly afraid of breaking their aesthetics, this being why they value their masters' lives above their own.
While demons are physically capable of consuming human food, as shown through Sebastian's sampling of lemonade, cake, curry, and wine at various points in the series, it seems that only human souls are able to satiate their hunger. William states that refraining from consuming souls can leave demons "starving" to the point that they cannot help themselves. All the same, demons do seem to be able to be afflicted by problems similar to humans, such as when Sebastian complains of heartburn after drinking the lemonade that contained MSG instead of sugar.
Demons are said to be "devoid of love" and lacking in convictions and loyalties. Nonetheless, they have their principles, which include following their master's orders and maintaining the integrity of a contract.
Demons also seem to have a long-standing hatred of Grim Reapers, which is illustrated through Sebastian's interactions with William T. Spears and Grell Sutcliff. Grell sums this up by saying, "If the job of a Grim Reaper is to recover the souls of the dead, then demons can be likened to pests that snatch these souls away just for the sake of devouring them." Sebastian, on the other hand, calls their use of the Cinematic Record "just a terrible interest" and the worst sort of spying activity.
However, it is understood that, as far as demons go, Sebastian is not particularly harmful, as William states that he is "slightly better" than the others and that he's "not the type to feast."
- Main article: Faustian contract
In order to create a contract, a human has to summon a demon. This can be done either intentionally or accidentally. A contracted person receives a mark of their contract somewhere on their body, with a corresponding mark appearing simultaneously on the demon.
Demons seem to be independent operatives. They contract on their own behalf, and there is no suggestion that they serve any supreme power nor any evidence that such an entity exists in the series (though there is a reference to God).
Demons are shown to have a variety of abilities, including:
- Body manipulation: Sebastian is shown to be able to twist someone's arm just by twirling his finger.
- Enhanced endurance: While fighting Grell Sutcliff, who was wielding his powerful Death Scythe, Sebastian was able to sustain multiple injuries from him and continue fighting while protecting Ciel. Sebastian is also able to survive attacks that would kill humans, such as when he was shot in the head and chest multiple times by Azzurro Vanel's men, get stabbed, slashed and even an arm severed yet continue on as if nothing had happened. However, more powerful weapons such as a Death Scythes can significantly weaken or even kill a demon.
- Enhanced senses: Sebastian is able to find humans based on smell—including ones he has never personally met—, is able to deduce if they are dead, alive, or injured and be able to discern if someone is a human or a member of another race, as shown when he quickly identified Agni as a human rather than a Grim Reaper, or even another demon. Sebastian states that he is able to pick up any kind of scent. He was easily able to hear the rifleman who attempted to kill him, his abilities extending to the point where he could see each and every person who escaped from Violet Wolf after it caught on fire.
- Enhanced speed: Sebastian has demonstrated the ability to outrun a car that was attempting to speed away from him. He is also able to block or dodge objects, such as his sword fight with Agni. He is also able to run to places in one hour that would take a day to get to by coach or railroad and outrun a moving train.
- Enhanced strength: Demons are shown to be vastly stronger than humans.
- Fire control: By revealing the sign of his contract, Sebastian is able to set an entire mansion on fire from three small candles. It is unknown if this is an inherent ability or if it is a demonic ability derived from the Faustian contract.
- Immortality: Sebastian has stated that demons can live forever, and they can heal/regenerate from any injuries, even if their entire bodies are destroyed. However, they can be killed by a Grim Reaper's Death Scythe.
- Levitation: It is shown that, when the first time Sebastian encountered Ciel, his feet weren't touching the ground.
- Manifestation: Sebastian was able to recreate Phantomhive Manor, including the building itself and all of the furniture and decorating, in a matter of three hours. He was also able to create a feast in an instant, as well as fill an entire tower of champagne glasses using far more champagne than a bottle could possibly have held. Sebastian also easily repaired Ciel's ring after Elizabeth destroyed it during a tantrum. Demons also seem to possess a great capacity for learning and retaining information, as shown with Sebastian quickly learning and excelling in cooking, art, language, music, etc.
- Manipulation: Beyond making contracts for a human's soul, Sebastian has offered to make other deals or attempted to charm and manipulate people in order to get what he wants. William has called this the "signature" ability of a demon, and states that it works by "deceiving prey with sweet words and dragging them into darkness." While ineffective against William, he later uses this ability to get Beast, who initially disliked Sebastian, to reveal important information.
- Mind manipulation: Demons are able to trick the minds of people into perceiving things or persons which are not there.
- Soul sensing: Demons can pick up signs of human life by sensing the humans' souls at long-range.
- 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. This, however, could simply by a metaphor, as Grell is known to speak in extremely unorthodox language patterns.
- Transformation: Sebastian has demonstrated that he can switch between his human form to his true demon form and back quickly. Furthermore, it is implied that Sebastian can change his appearance at will, be it humans or creatures. He can also shapeshift his own body to weapons during the creation phase of his contract with Ciel. He was also able to take away his skin conditions from the werewolf's miasma by swiping his hand across a mirror.
Known contracted humans
- Sebastian has been seen wearing a rosary, which would suggest that sacred objects (such as crucifixes, holy water, etc.) do not have a negative effect on demons.
- Demons are incapable of turning back time or resurrecting the dead.
- The second season introduces five more demons beside Sebastian: Claude Faustus, Hannah Annafellows, Timber, Thompson, and Canterbury. At the end of the season, Ciel Phantomhive is turned into a demon.
- In the first season, Angela Blanc comments that a demon being starved causes them to feel more pain than normal when physically injured.
- Claude has once stated that a demon's—most likely those that nearly exclusively form contracts—life was to "cultivate a soul and then feast on it", cultivate within the context taken to mean shaped.
- Demonic conversion: Demons are shown to be able to convert humans into demons. Alois, while possessing Ciel, ordered Hannah not to allow Claude or Sebastian to obtain Ciel's soul which resulted in Ciel being reborn as a demon, waking with the signature pink-slitted eyes and withstanding a direct attack from Sebastian. It is not clear if this ability is available to all demons.
- Soul manipulation: Aside from consuming souls, demons can also manipulate human souls to a certain extent such as implanting them in physical objects. For example, Claude was able to steal Ciel's soul and implant it in his personal ring which he also stole. He later implanted the soul of Alois into the latter's own personal ring after killing him.
- Transformation: It is questionable as to whether this is an ability all demons have. Apart from Sebastian, only Claude displays the ability to transform at will: He is able to change into the form of a spider, once had an inhuman tongue for a brief time, and has spun and shot webs while still in human form.