PERSONALITY:
"Many people, meeting Aziraphale for the first time, formed three impressions: that he was English, that he was intelligent, and that he was gayer than a tree full of monkeys on nitrous oxide. Two of these were wrong; Heaven is not in England, whatever certain poets may have thought, and angels are sexless unless they really want to make an effort. But he was intelligent. And it was an angelic intelligence which, while not being particularly higher than human intelligence, is much broader and has the advantage of having thousands of years of practice." (page 147)
Aziraphale is a very complicated character in that he isn't your average, stereotypical, sweet and kind angel. That isn't to say he isn't sweet and kind; he willingly gave up his flaming sword to Adam and Eve and was willing to lie to God about it later. Aziraphale's intentions are good, and he always does his best to make sure his intentions are good.
He also drives Crowley crazy with his stubborn insistence on always doing the Right Thing. When Crowley runs down Anathema, Aziraphale not only miracles the girl and her bike back to health, but insists that Crowley give her a ride back home, despite the fact that Crowley knows he and Aziraphale have more important things to do. Aziraphale's natural sense of moral obligation even restrains him when Crowley isn't involved. For example, he refuses to join the armed forces of Heaven right before the Apocalypse so that he can stay behind for a few minutes and "clear up a few business matters," even though since Armageddon is about to occur, it's a completely futile and pointless exercise. However, Aziraphale insists that "as a reputable businessman" he should do the right thing.
There are times throughout the course of the book where Aziraphale displays characteristics that are, in a way, un-angelic. One thing to keep in mind is, Aziraphale's motivations for preventing the Apocalypse could be read as purely selfish - he likes the Earth and its pleasures, that's why he doesn't want the Earth destroyed. To the customers at his bookshop he gets cold and rude because he doesn't want to sell any books. He's materialistic, has limited patience, and is even conniving in his own angelic way (i.e., the Arrangement, the "Hey, God? About that flaming sword? Yeah, uh... I kind of lost it..."). Aziraphale also admits that he helps support terrorists as long as he can justifiably call them "freedom fighters," and he further supports gun ownership rights; "(Guns) lend weight to a moral argument. In the right hands, of course," he explains to Crowley. However, Aziraphale does often feel guilty after he does something bad. He tries extremely hard to always do what is right and good.
In some ways, Aziraphale is more cynical than Crowley is with his outlook on humanity. His cynicism is hidden, in a way, under layers of angelic oh-I-can't-say-that-ness that's less honest and blunt as Crowley's cynicism. Aziraphale can be scarily jaded that contrasts sharply with his default stereotypical angelic nature (and the fact that he's supposed to be the good guy); at least Crowley is honest about it, whereas Aziraphale is all goodness and light until he sucker punches you with this really deep cynicism and ow, that came out of the left field. Except it didn't, really.
On the other hand, Aziraphale has that old-fashioned English politeness covering up what he thinks of things- the politer he is, the more cynical he is. Honestly, who can capture the meaning of "Oh, for fuck's sake, grow up and act your damn age, don't you think it's just slightly beneath the dignity of even a fallen angel to muck around scaring ducks?" into one little murmur of "Really, my dear?"
(Though probably his most cynical comment would be that bit where he suggests to a television audience that "You might just as well send money to a Satanist hotline to cover your bets.")
Aziraphale is also vain (think of his expensive clothes and expensive manicures, and he's more than a little sensitive about his weight) and more than a little petty (his comment to Shadwell "Not just a Southern pansy" says that it's been on his mind for a while).
Though ultimately, both Aziraphale and Crowley are slightly tinged with the other's personality. In a way, they wouldn't be quite as likeable if Aziraphale was a righteous prat and Crowley was an evil bastard.