The angel who guarded the gates of Eden with a flaming sword, Aziraphale has lived among humans for thousands of years, and has become quite fond of them as a result. Due to his friendship with Crowley, he's not quite as pure-hearted as he should be.
Asexuality: The way it's phrased is a little ambiguous, but the implication is that being sexless unless he really wants to make an effort means he doesn't normally have a sex drive or the parts to act on it with.
Memetic Outfit: A large chunk of the fandom seems to agree that Aziraphale wears argyle sweater vests and/or tartan. There are also some who give him glasses in fanworks, although this hasn't been mentioned at all in the book.
Crowley was the Serpent who tempted Eve in the Garden of Eden. Having lived on Earth for past six thousand years, he's become rather fond of humanity. His friendship with Aziraphale means he's not quite as evil as a demon should be, though he was never very evil to begin with.
Even Evil Has Standards: After getting praise from downstairs for his work in Spain, despite not having worked there recently, he went there to find out exactly this thing called Spanish Inquisition the humans created all by themselves without any hellish influence was. After finding out, he drank himself into a stuporto forget the whole thing.
Evil is Cool: Has style, all the latest gadgets, and a flashy car.
Laser-Guided Karma: His beloved car ends up totaled when he has to drive it through the M25... after said highway starts emanating a bizarre and dangerous energy field. Which he set it up to do.
Lovecraftian Superpower: Scares off the paintballing corporate commando by turning into something dreadful... "I think the maggots were a bit over the top, myself."
Meaningful Name: Crowley, a reference to occultist Aleister Crowley. It also references his original role as the serpent ("Crawly") that tempted Adam and Eve.
What Do You Mean, It\'s Not Heinous?: Crowley's reaction to his immediate superiors being rather underwhelmed by his demonic influences (i.e. traffic jams and telemarketers) on the mortal world.
Well, it's really more of an example of quality vs. quantity, with his superiors focusing on the former and Crowley the latter.
The Woobie: "He hadn't meant to Fall, he'd just gotten in with the wrong people". Getting dosed with a water hose while trying to find Aziraphale - his only friend in the world - in the latter's burning bookshop. Being terrorized by the Dukes of Hell over the radio. Remembering what the Antichrist can do to him just by looking. Pick one.
Properly Paranoid: At one point, it's mentioned that Hastur is paranoid, which is in fact a very reasonable thing since as he is a denizen of Hell where everyone really is out to get you.
The Glasses Come Off: Subverted: when he takes his glasses off, he ends up looking less handsome because he keeps running into things and ends up covered in bruises and bandages.
In the Blood: Newt's ancestor was the one who laid the torch to Agnes Nutter (who was, in turn, Anathema's ancestor).
Eerie Pale-Skinned Brunette: While not that eerie once she becomes a major character, she has the coloration and she does start off as a bit of a Strange Girl.
Try to Fit That on a Business Card: The Antichrist the Adversary, Destroyer of Kings, Angel of the Bottomless Pit, Great Beast that is called Dragon, Prince of This World, Father of Lies, Spawn of Satan, and Lord of Darkness.
