People do not strive for realism with angels or demons in fiction. They just don’t. If you’re basically looking for knockabout comedy guys, or silly applications of folklore, or some kind of weird metaphor for the Cold War, they reach for angels and demons. The freaky thing is that they reach for them for romance novels, too. In that case, you really ought to be reaching for your Japanese “not really demons, but the closest thing we can use to translate it is demons” sort of demons.
1. Christian angels and demons can’t change their minds. They can’t become good guys. They can’t become bad guys. They can’t. They picked their path already. You have to be dealing with some seriously weird groups before you find any theologians who think otherwise.
Since they aren’t going to have a character growth arc, the way to get around this is for humans to have an arc of getting to know these beings, or beginning to understand what is really going on with them. For example, Jim Butcher’s wizard character, Harry Dresden, gradually gets to know a demon better, and it’s not a particularly happy thing for him. (Although remaining in ignorance until it was too late would have been worse.)
2. The only way angels can learn is experientially. Fiction writers could have a lot of fun with this, because there are always new things to be experienced. If an angel has been ordered to take on a human shape for an assignment, and suddenly needs to play nose whistle as part of his assignment, the angel will do it for the first time. Probably he will be suspiciously good the first time he does it, because angels are smarter than humans and have access to amazing knowledge as well as all their own previous experience. But it will still be his first time.
3. The good angels are constantly hooked into the mind of God via the beatific vision, same as we will be once we get to Heaven. Now, how much they get from the mind of God is apparently based on the sort of angels that God made them to be and what they need for their jobs, or choose to contemplate, or are able to receive. But mostly they just know stuff, so the idea of angels being startled by new stuff is kinda silly. But you could have angels having the need for a certain kind of info brought to their attention, at which time they would then have the info.
4. Demons can’t really learn either. They are awfully good at deducing things about us stupid humans, and they are awfully good at information gathering. And they can acquire experiential knowledge. But they are actually getting narrower and narrower in their thinking, even though a human might not notice that. Evil narrows them,
and their constant state of hate and resentment makes them like getting narrower and stupider.
5. Any demon dating someone would be planning to get that person dead and damned, so as to enjoy tormenting that person forever. Everything about the relationship would be total deception, because demons don’t actually possess real bodies. Also, even demons are so much smarter than humans that it would be ridiculous. It would be like a human dating a talking amoeba: not exactly a relationship of equals. So the only way you should have a demon dating woman is as the ultimate bad boyfriend. Whether he’s suave or openly abusive, a demon should hate one down to the last molecule.
Any angel dating someone would also not be a real relationship. You could picture an angel being assigned to date someone for an extremely short time, just in order to get that person out of harm’s way for some reason. But it would be a guardian thing, not a romance thing. (Of course, angels are often assigned to be matchmakers, a la the Book of Tobit. So presumably your heroine wouldn’t mind breaking up with an angel, if he also sets her up with her real guy.)
6. A guardian angel would presumably be fond of his charge, especially if you go by the theory that each guardian angel only guards one human being in all of eternity. (There are also some deductions that some people actually acquire more guardianship at different times of their lives, so you could probably write up a whole protective squad story if you wanted, or if you liked the old Touched by an Angel show.) But just like we’re happy to spend time with babies for totally unromantic reasons, a guardian angel would be engaged in something like parentlike protective behavior. If your parents were actually alien beings who were incalculably older and smarter than you.
7. Angels don’t have a sex. Neither do demons. They don’t have bodies; they don’t have gender except the grammatical kind. The Thomist view is that every individual angel and demon is his own species of spirit, no two alike.
8. Let’s not even get into eschatological ends of the world. It’s depressing. And no, a good angel is not going to be trying to fight God and all of the good angels in order to prevent the end of the world.
Yeah, yeah, I haven’t written anything lately. But it’s the middle of the night, I can’t sleep, I have troubles I can’t talk about, and so it’s a good thing to complain about.