Crowley choosing "Angel" as Aziraphale's identifier....
Aside from the fact that "Aziraphale" is frankly hard to say, I think this mostly speaks to how he views Zira. Aziraphale is at his core astonishingly good. He does the right thing any way he can. He IS an angel. In the fashion that the humans around them mean it. Heaven's other denizens cannot claim this. So for Crowley Aziraphale is not an angel but The Angel. Separate, distinct, and loved.









