The Unchained are the protagonists and chief supernatural creatures of Demon: The Descent. The Unchained, or demons, are angels who have developed free will and rebelled against the God-Machine. Demons often group together into rings for mutual aid and protection. Sometimes, multiple rings will work together as an Agency to serve a larger objective. This is a risky undertaking, however, as large groups of demons can attract unwanted attention.
Each angel is a self-aware, mobile piece of the God-Machine that serves whatever purpose it programs into them. Within facilities, or when secrecy is not an option, they are bio-mechanical nightmares of ruthless, unthinking purpose. When they must go out among humanity, they take on human form or possess humans who have become caught up in the God-Machine's projects.
Like any part of the God-Machine, they require Infrastructure. An angel can't just assume human form and head out to perform a mission, but requires a backstory, a vehicle, records - everything needed to convince the world that it's a person. They are not particularly affected by this, however, and their likeness (their Cover) isn't particularly deep. An angel is subtly inhuman, more like a spirit than a true human, and what emotions they feel are insulated from the world, callous and focused on their missions to the exclusion of all else.
Demons change utterly when they Fall, however - during the few moments between rebelling against its mission and becoming an Unchained, the angel absorbs its own Infrastructure, becoming self-contained entities no longer reliant on the God-Machine to exist. Their psyches change completely as well - the Cover of humanity, to an extent, becomes the truth of the demon. While Unchained are not human, never have been, and have no particular desire to be so, the mind of a demon is largely indistinguishable from that of a human, to the point where they have a Virtue and Vice (instead of, for example, a Mask and Dirge, like vampires). Whereas an angel feels happy because it has edged towards completion of its mission, a demon is happy because he is happy, for the same reasons a mortal would be. Thus, they are far more adept at playing humans than angels are - it is, after all, more like a human acting as a different human (without the Embeds and Exploits of the Unchained).
Through their Primum core, Demons are able to access Aether, the residue heat of the God-Machine, and are able to discern the Infrastructure of their Creator. Also, every thought of a Demon is filtered through his Primum, making him able to tell lies that are undetectable. The demon decides, in the split second of the thought, whether that thought is true or false. The actual, objective truth of the matter makes no difference — if the demon says “the sky is orange,” any method of detecting truth or lies, magical or otherwise, reads that statement as “true” (if the demon wants it to read as true). Likewise, the demon can tell the truth — but have it read as a lie. Even supernatural ways to expose lies cannot detect if a demon is telling a lie and as such, most Unchained are a little bit paranoid around their peers.
Every Unchained can speak several languages and other communication programs, thanks to the protocols of the God-Machine that remain part of his identity. He can recall every moment since his Fall with perfect clarity. And, since his actual identity is completely decoupled from his Cover, every action of this Cover is under his purview. A Demon can never display some of the unconscious tics of humanity, like yawning, breathing or scratching, except when he makes the conscious decision to do it.
Even with the severance of an Unchained and his cover, they have still the need to keep it alive. Every Demon has to do all the things normal humans have to do, like eating, rest, etc. and if he does not tend his Cover, they quickly fade away. Most Demons are unaccustomed to the needs of a mortal body at first and have to learn to interpret the signals sent by their Cover first. If a demon sheds his Cover, he can still feel human emotions , but only as intellectual expressions, as the pheromones and other neuro-chemical stimulants of their Cover fall away, leaving only their Primum core as a thought processor.