The character's name is a reference to the occultist Aleister Crowley, which is played on in the book - although his initials are revealed in an early scene, it is not until he answers a phone call much later on that his first name is revealed to be Anthony. This name is itself a reference to Anthony Rowley, the frog in the song 'Frog he would a-wooing go'. The pun is made explicit by the inclusion of the sentence '"Hey-ho," said Anthony Crowley' in the book, in reference to the chorus of the song.
His "real" name, the one with which he signs infernal contracts, is described as "a complex, wiggly sigil."
In the very beginning Crowley appears as a snake, but he uses his "favorite shape" for the majority of the book, described thus: "Crowley had dark hair and good cheekbones and he was wearing snakeskin shoes, or at least presumably he was wearing shoes, and he could do really weird things with his tongue. And, whenever he forgot himself, he had a tendency to hiss. He also didn't blink much."
He always wears sunglasses, with good reason; his eyes are yellow, "with slitted vertical pupils". Like Aziraphale, his "true shape" has wings, which are usually held in.
The book states, "Contrary to popular belief, the wings of demons are the same as the wings of angels, although they're often better groomed." Nevertheless, Crowley has been drawn with stereotypical bat wings (as well as horns and a tail) in official art. Other interpretations sometimes give him angel-style white feathered wings, as the book states that, because they are both of "angel stock," there is no apparent difference between the wings of angels and demons.
Within the book Crowley momentarily transforms into "something dreadful" to intimidate an attacker. (The only hint at its appearance is Aziraphale's comment "I think the maggots were a bit over the top, myself.")
Originally named Crawly, he was the serpent who tempted Adam and Eve with the Fruit of the Tree of Knowledge of Good and Evil. He soon tired of the name, stemming from the fact that "it just wasn't 'Him'".
After changing his name to Crowley, he went on to become one of Hell's agents on Earth, under the guise of a stereotyped yuppie. He is fascinated by humanity's ability to do worse things to each other than the legions of Hell could imagine (largely because the legions of Hell have no imagination). The personal achievements he himself is most satisfied with include Welsh Language Television, game shows, value-added tax, Manchester and the M25 London Orbital Motorway.
He regularly pulls pranks and causes small annoyances to people, on the belief that they will become angry and cause small annoyances of their own choosing, in a domino effect-- thus he won't have to work so hard. "Thousands and thousands of souls all got a faint patina of tarnish, and you hardly had to lift a finger." He wipes a girl's contact list on her cellphone, at one point, and in another case, causes a city-wide tie up of "every portable telephone system in Central London for forty-five minutes at lunchtime."
Crowley is unusual among demons in that he, as he puts it, "hadn't meant to Fall. He'd just hung around with the wrong people." He disapproves of many things about Heaven, but isn't very enthusiastic about Hell either; ultimately he prefers Earth to either one. In the book's list of dramatis personae, he is called "An Angel who did not so much Fall as Saunter Vaguely Downwards."
Although he is responsible for transporting the infant Antichrist to the hospital where he will be switched with another infant, precipitating the Apocalypse, Crowley decides he would rather have humanity continue than see either Heaven or Hell win. When it transpires the Antichrist was a different boy all along, he worries that his superiors would do all of the things he described in his reports on the Spanish Inquisition, first individually and then all at once. His eventual attempt to thwart the Apocalypse puts him in Hell's bad books ("not that Hell has any other kind").
As the end of the world approaches, Crowley finds himself on the run from fellow demons, as well as Dukes of Hell, Hastur and Ligur. He dispatches Ligur with an emergency supply of holy water (which he treats with the same care as a human might treat an armed bomb), and manages to get Hastur temporarily trapped in his ansaphone. (Form is immaterial to demons; in this case, Crowley leads Hastur on a chase through his telephone line.)
Crowley has a strong friendship with his opposite number, the angel Aziraphale. The two often meet up to compare notes, much like Cold War agents who find they have more in common with their immediate opponents than their distant superiors.
There is a lot of overlap between the jobs of the two; for example, both he and his supposed Enemy reported Milton Keynes to be a success. (Crowley is, however, single-handedly responsible for Manchester and Glasgow.)
After six thousand years together, the two have come to an Arrangement by which they occasionally help each other to do their work, saving everybody time and travel. They've also rubbed off on each other, a bit. Crowley states to Aziraphale, at one point, "Oh, all right. No one's actually going to get killed. They're all going to have miraculous escapes. It wouldn't be any fun, otherwise."
Aziraphale tells Crowley he had "known, deep down inside, that there was a spark of goodness in you." To which Crowley responds that he has "known that, deep down inside," that Aziraphale was "just enough of a bastard to be worth liking."
Crowley's most prized possession is a black 1926 Bentley, with one owner since new: Him. Although his taste in other gadgets (pens, cell phones) tends towards the modern, he has never updated the Bentley, possibly because its performance relates more to Crowley's expectations of it than any mechanical function. Notably, he has refueled it only once, in 1967, and then simply because he rather fancied the free James Bond bullet-hole-in-the-window transfers.
Despite being a Demon and not actually requiring it, Crowley is very fond of sleeping. He enjoys it so much that he slept through the entire of the 19th century, although he did have to get up in 1832 to use the bathroom.
Judging by the labels on the cassettes in his car Crowley enjoys classical music, presumably on the basis that - as he notes to Aziraphale - the only composers who ever got into Heaven were Elgar and Liszt. He also has a collection of Soul Music - "real Soul Music. James Brown wasn't in it."
He owns a stylish London flat, although he doesn't really 'live' there in any sense. He does, however, pay attention to the houseplants, which are glorious and lush - largely because he terrifies them into keeping up their size, or else. Crowley owns the original sketch of the Mona Lisa, which apparently da Vinci infinitely preferred to the painting, claiming he could never quite get the smile right. He also inspired da Vinci to design helicopters.
Although no sequel to Good Omens has been written, there is some information about Crowley's life beyond the book. The HarperCollins website features a list of his New Year's resolutions