Crowley is a demon. In the biblical sense. A fallen angel, enemy of God *, servant of Satan and a cog in the machine that encapsulates all the unholy forces of Hell. One might even describe him as the worst possible demon you could meet.
Because he's very, very good at his job.
This is because Crowley, unlike many demons (and most angels for that matter) has an imagination. He understands all the little idiosyncrasies that make people tick. He knows exactly how to get the kind of reaction (usually negative) out of people that he needs without having to resort to planting one of the seven deadly sins directly in their head.
Crowley understands that people are practically chomping at the bit to shoot themselves in the foot, which makes his job tempting them to do so almost absurdly easy.
This is most evident in his methods. He doesn't go big (planting lust in a priest or greed in a politician), but broad. For example, he cut the London mobile phone lines and bam. A whole afternoon of hundreds of thousands of people taking their terrible moods out on each other. He also designed the M-25 to be built in the shape of Odegra so that traffic would work like water on a prayer wheel, dispensing a lot of low-grade evil into the atmosphere.**
And this leads into one particularly peculiar quality Crowley possesses - an abiding sense of fairness. This is odd because Crowley knows exactly how absurd this is. He knows there's no such thing as 'fair'. Life's not fair, the universe is not fair, God certainly isn't fair.
But Crowley? Crowley is fair.
As far as he's concerned, there should always be a choice. A CEO doesn't have to take it out on his secretary just because he's been mildly inconvenienced by something out of everyone's hands - but if he does? Well, he has no one to blame but himself for the fresh coat of evil tarnish his soul gets for it.
You see, Crowley never got that; a choice.
As he tells it, he only ever asked questions and hung out with a bad crowd - and then the next thing he knew, he was plummeting head-first into a fiery pit of boiling sulfur. And this doesn't seem like a case of an unreliable narrator either - Crowley seems genuinely hurt and bewildered about his fall. He's mystified by God casting Adam and Eve out of the garden over a piece of fruit, calling it an 'overreaction'. He's outright indignant when Aziraphale tells him that God plans to wipe the locals in a great flood. (And more book canon than show canon, he was definitely not a fan of what happened in Sodom and Gomorrah).
So when it comes to making choices, he likes to give the humans more than a fair shake. After all, actual evil doesn't come from making desperate choices when you're caught between a rock and a hard place, but the banality of day-to-day life. It's the little things.
Contrary to his work, Crowley is not actually a bad person. Annoying? Yes. Absolutely a trickster overfond of juvenile pranks. Mercurial, certainly. He has a sort of frenetic energy that can be very hard to deal with and he questions absolutely everything (perhaps the double-edged sword of having an imagination. On one hand, he can devise elaborate schemes - on the other, he tends to overthink situations that might have perfectly simple solutions). He can be genuinely charming if he wants, even gallant at times - he does seem to delight in getting to play the role of dashing hero
He also lacks a certain sadistic malice most demons possess. He just doesn't have the stomach for it. Truly evil acts tend to throw him for a loop; he's shown to vehemently object to (He still took credit for the whole debacle though - he is nothing if not an opportunist - especially if it means less actual work for himself).
One might call this repulsion to horrors (divine, infernal, or just plain human) 'compassion'. One might also be thoroughly rebuked with a lot of furious stammering and other incomprehensible protests. He's just got a bit of a soft spot for the mortals. Been on Earth and around humans for far too long - he's practically gone native. And he quite likes it that way. He likes humanity and all their clever machines, the countless inane ways to waste time, their art, their films, and especially their music.
*[ Broadly speaking. Crowley's feelings on Her are a little more complicated.]
**[ Despite these being some of his crowning moments of ingenuity, both also managed to bite him in the ass to varying degrees of severity. It's probably safe to say that these are far from the only times Crowley has been hoist by his own petard. ]

Crowley looks to be around his mid-to-late forties, although he is much, much older than that.
He keeps up with the trends of the times, usually favouring black attire, or very dark colours/earth-tones. Tends towards presenting himself as whatever passes for a rogue-ish rake of the time period.
He's never without a pair of sunglasses to hide his eyes - they can go a bit odd if he forgets himself. He was once a snake, still kind of does the tongue thing, but definitely seems to like his human form (the book describes it as his 'favourite' form). Walks like the aging rockstar he wishes he was.
Largely used for day-to-day convenience Do not actually have to be malicious in nature - as per his Arrangement with Aziraphale, Crowley often takes on both blessings and temptations (and visa versa)
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SKILL TITLECrowley is effectively immortal - his physical form can be destroyed fairly conventionally (barring a few exceptions, if it would kill a human, it will discorporate him.) The well-placed demonic miracle may make things a bit tricky - but mostly "death" is just really, really annoying inconvenience to him. That being said, he is shown to be vulnerable to consecrated ground and will be killed completely dead by holy water. One could probably extrapolate from this that he's vulnerable to anti-evil measures from other denominations.
Crowley can put people into a temporary trance where they tell him the truth. Unfortunately, telling him the truth doesn't necessarily mean telling him anything useful. Crowley can sense desires tied to sin - envy, lust, avarice, pride, wrath, sloth, gluttony - and will alter a situation to give people exactly what they want. Hastur and Ligur are able to plant temptations directly into the minds of mortals. It is possible to extrapolate that this is just a general thing demons can do since they seem to expect the same methods from Crowley. That being said, Crowley is never shown to use this ability so either he can't or won't.
- **Bolded** text falls into the realms of headcanon and conjecture.
- ...There wasn't a a whole lot going on. That is until the Almighty flicked the switch and the universe began. With all that loose matter floating about, She probably couldn't be arsed to sort it all by Herself, and so she made some angels to handle the minutia of the Great Plan while She worked on Her passion project. One of these angels was Crowley. Of course, he wasn't called Crowley back then, nor was he called Crawly. It's not entirely clear who he was back during his brief stint as an angel, but it doesn't seem like he was anyone of particular note. He helped build stars and nebulae and once that was done, he returned to Heaven to make some friends and find out what all this Earth business was about.
- Unfortunately, Crowley was not a particular good judge of character, and with a head full of questions and a mouth with zero filter, he fell in with Lucifer and his posse. With God not being one to like Her loyal servants asking "why" all the time, it's pretty clear to the casual observer what was going to happen next. Crowley was not a casual observer, however, and getting jettisoned into a pool of boiling sulfur took him rather by surprise.
- Once all the kerfuffle of Creation and Celestial Civil Wars had settled, Crowley (known then as Crawly) drew the short straw and was shunted off to Earth to make as much trouble for the shiny new humans as he could. This suited him just fine, because Crowley fit in with other demons about as well as he'd fit in with the angels. You probably know how the rest of the story goes - snake talks the first woman into eating the fruit of knowledge, and God gets rather tetchy about the whole thing. What happened after is... less well-known. Crowley struck up a conversation with Aziraphale, the angel of the eastern gate, who was in quite a state since he'd just given his flaming sword away to Adam and Eve out of concern for their well-being and thus an unlikely friendship was born.
- Crowley spent the next couple of millennia on Earth puttering around and making trouble (but mostly just slacking off and taking credit for all the trouble the humans made themselves). He'd always liked a bit of mischief, but his heart was never in the whole 'evil for evil's sake' agenda Hell was pushing. About a thousand years after Eden, Crowley met up with Aziraphale again in Mesopotamia. Not as pleasant as their first encounter as God was just about to drown everyone in the area, including children which Crowley found to be somewhat objectionable.
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