An angel of Heaven. There are three things people always assume upon meeting Aziraphale: 1) that he is English, 2) that he is very intelligent, and 3) that he is gayer than a tree full of monkeys high on nitrous oxide. However, heaven is not in England and angels don't have genders unless they try, very, very hard. But one out of three isn't bad.
Aziraphale was the original owner of the flaming sword, and was meant to use it to guard the eastern entrance to the Garden of Paradise. However, pitying Adam and the pregnant Eve, Aziraphale gave the sword to the humans. It finally ends up in the hands of War, one of the Four Horsemen of the Apocalypse.
Aziraphale has been a 'field agent' for Heaven for thousands of years, and due to being more in touch – and having more in common – with his opposite number than with his colleagues in Heaven, he has developed a rather flexible Arrangement with Crowley.
He runs a bookshop in London, where he only sells a book when he absolutely must. He is an expert on prophecy books, and possesses a few copies that have inscriptions dedicated to him.