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 and all those years among free-willed mortals, he's not quite as pure-hearted as he should be.
Ambiguously Gay/Mistaken for Gay: It's stated that pretty much everyone's first impression of Aziraphale, among other things, is that he's "gayer than a treeful of monkeys on nitrous oxide". Him hanging around with Crowley all the time certainly doesn't help.
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.
Heterosexual Life-Partners: With Crowley. (Although "non-sexual" might be a more accurate term, since the narrative makes it clear that neither of them even have a technical gender, let alone sexual orientation)
Innocent Bigot: Despite all evidence to the contrary, and despite basically considering him a friend, he's still convinced that Crowley is incapable of feeling love or loyalty, simply because he's a demon. To him, it's not prejudice, it's just a fact. In complete fairness, it's supposed to be, but Crowley's been around humans too long.
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 about this thing called the Spanish Inquisition that the humans created all by themselves without any hellish influence. After finding out, he drank himself into a stupor.
Evil Is Petty: The type of evil he likes to inspire in people; nasty, petty, small-minded little cruelties fuelled by bad tempers and off days. It doesn't require much work on his part to get to thousands in this way, and - in his opinion, the best part - people will come up with the evil themselves. People are so much more creative than demons.
Felony Misdemeanor: Crowley's reaction to his immediate superiors being rather underwhelmed by his demonic influences (i.e. traffic jams and telemarketers) on the mortal world. It's something of an example of quality vs. quantity, with his superiors focusing on the former and Crowley the latter because Crowley believes the former is outmoded.
Note that he only really tries to drown ducks when Aziraphale is around to tell him he has to stop. This seems to run on the same sort of logic of "well, if we go out to dinner together, we're both technically stopping the other from doing good/evil deeds, and therefore doing our jobs"
Only Sane Employee: He's seemingly the only demon who considers planning in the long-term, believes subtle acts of mass-corruption are the way forward and thinks that the Apocalypse is a bad idea for business.
He also tried to introduce technology to Hell, thinking it would be a better way to contact him. Unfortunately, instead of adopting telephones like he'd wanted, they instead decided to contact him through his car radio, satanically twisting whatever he was listening to at the time.
Partial Transformation: Despite being an accomplished shapeshifter, even when he's in human form he likes to retain Hellish Pupils and it's implied that his trademark snakeskin boots are actually just his feet.
Celestial Body: His wings are described as black holes cut out of space with a scattering of lights gleaming in the darkness, which might be stars or might be something else entirely.
Don't Fear The Reaper: He's not as kindly as other personifications of death created by the same authors, but he's not truly malicious or destructive, and only seems to be going through with the apocalypse out of a sense of duty. Once the other three are defeated, he leaves without putting up any fight. In the epilogue, he's briefly seen feeding ducks in the park.
Token Good Teammate: He's more in touch with humanity than his comrades and by many leagues the most powerful. The evils of War, Famine, Pestilence and Pollution might someday come to an end, but Death has always walked in the footsteps of life and always will.
Cool Sword: Not fancy, but it looks well-designed for the sort of thing that swords do, which is killing a very large number of people. And it's not just any sword, it's the one Aziraphale gave away. You hand a divine weapon to the two first humans, you don't go expecting a personification of peace to wind up wielding it, do you?
Corrupt Corporate Executive: Though there's no sign his business practices themselves are more than typically evil, the products sold are designed more for inflicting suffering than turning a profit, although he may be the only one to realize it.
Insult Backfire: He loves it when people drop garbage all over the place, actually.
The Pig Pen: Anything he touches becomes encrusted with corrosion and filth.
Pretty Boy: Described as looking like a beautiful, tragic Victorian Romantic poet before the tuberculosis really starts setting in.
The Reveal: For whatever reason, the fact he's Pollution and not Pestilence is made intentionally ambiguous until the big confrontation. For example, when he signs for the parcel containing his crown, it's noted that his signature is so messy that, while it definitely starts with "P", it's difficult to tell whether it ends in "-ence" or "-tion".
Tagalong Kid: The youngest of the horsepersons. Pestilence retired after the invention of antibiotics.
Technopath: One of his special gifts is making machines do just what he wants them to do, regardless of fail-safe interlocks or foolproof backups. What the machines do isn't even that consequential, usually, unless you discover they're the other end of a long chain of such inconsequential events that lead to toxic waste disasters and oil spills.
Awesome, but Impractical: She was the only perfectly accurate prophet in all of history. However... she can only see the things that happen to her descendants. Not only is this pretty useless to everyone else (for example, on 22 November 1963, she predicted a house in the British countryside collapsing, not the JFK assassination... her descendants could've been near that house, but they'd be nowhere near Dallas), it's also pretty useless to her descendants because the predictions generally have very little context. There's also the problem of her almost indecipherable wording choices, which lead to a high number of prophecies only being figured out after they happen.
Walking Techbane: To a truly spectacular extent. He's obsessed with electricity, machines and computers and desperately wants to be an expert - but his destiny has something different in store.
Chekhov's Skill: Or lack thereof. To prevent the Apocalypse through destruction of a computer, the way he manages to make it cease to function is to attempt to make it work better.
Epic Fail: Newt once tried to assemble a joke electrical non-working thingy designed for the most hamhanded tech-deficient beginner; if hitting the on-switch does absolutely nothing, that means it's "working". When Newt finished it and turned it on, it picked up Radio Moscow.
Aura Vision: An offshoot of her psychic powers. It's somewhat troubling for her when she can't see a certain person's aura.
I Know Your True Name/Meaningful Name: It's made clear with Dog that names influence what a thing becomes. It's more subtle in this case, but maybe if the book had gone on calling him 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, instead of Adam (which means Man) then he wouldn't have turned out so perfectly human.
You Will Be Spared: When his powers manifest, Adam decides to destroy and recreate the world, bringing about the apocalypse. However, he affirms that his friends and Dog will be spared, allowing them to each rule a portion of the new world.
One of the Boys: In reaction to her name, she had to grow up to be very tough. One character mentally describes her as "marginally female." It's strongly hinted that when she isn't around the rest of Them she has much girlier interests, which is something she won't admit to having.
I Know Your True Name/Meaningful Name: Adam assigning him a name is what gives him his true purpose. If he had been named Killer, or Walks-by-night, the hound's personality would've changed to match. But then Adam named him Dog, turning him into a fairly normal...dog.
The Mind Is a Plaything of the Body: His name may have been the final nail in the coffin, but the book makes it quite clear that changing his shape to fit Adam's description of his dream pet was the major contributing factor behind Dog's... doggishness.
Sealed Evil in a Teddy Bear: He's a hellhound and still marginally aware of that, but due to being forced into the form of a real dog he's far too easily distracted and excitable to do anything about it.