Now, there's no rule against it or anything; unless it's really offensive, you're not going to get in trouble for making your character the son of a fallen angel and a human who was possessed by a demon or whatever.
However, I'd like you to know that you, the average joe, don't know squat about angels, demons, or even spirits. You could ask (You know; like mature people do when they realize that they're not omniscient?), and I could tell you what little I know, but it still doesn't actually make sense for you to summon angels or the like. See, you're not God, not Satan, yada yada. You can't RP as them for obvious reasons. To have any sort of manipulation of such beings would be blasphemous, which is not only rude to those of us who know better, but makes no sense.
Think about it. You come up with some story by which your character was some great angel that decided to go awry and started doing all kinds of crap. Meanwhile, I read your very unoriginal story and know that it doesn't even pretend to be logical to those of us who know anything about the subject(s). So, even though you probably put some good work into it, it's just a disappointing story, not worth reading, for me. No, it's not just me, but, even if it was, it's still rather rude.
Of course, there's always the question of how to defeat or affect a "real" angel, demon, InsAngel, familiar, or whatever being your character supposedly is, despite your quite likely lack of knowledge thereof. Well, everyone keeps running into the question and wondering how they're supposed to logically role-play things out. It's not because we're all stupid; it's because your character doesn't make sense. It's just a pain to role-play with an angelic, demonic, yada yada character. Sure, it's been done just fine before, but that doesn't mean that everyone's going to agree that your theories make sense, especially when you're just pulling them out of your hat of social influence.
Like I said, there's no rule about it; if you insist on making a character that is supposed to be a [insert character race here] and don't follow the ideas that I'm about to present, fine; we're not going to hold it against you, and we'll just try to be logical about stuff.
So, let's say that you're reading this and get what I'm saying. Fantastic. So, you have this character that you want to use, but it's one with some sort of celestial theme or something. Basically, you want an angel character or the like, but you don't want it to be illogical and rude to other RPers. Well, lucky you: You have some options.
- A personal favorite is to take the word that you were going to use, such as "angel" or "demon" (or sometimes just another word altogether, such as any other themes that you may have been going for, the name, or whatever), and run it through a translator at least once or twice (partially depending on which translator you're using) and/or jumble the letters up. I recently posted an Al Bhed translator, right? Go try it. Here are a couple of results:
angel = yhkam ("ae-ha-kukahm")
demon = tasuh ("teah-seeoo-ha")
yhkam = orgys ("oh-ra-geae-see")
tasuh = dycir ("deae-kuee-ra")
Not a bad start, if I do say so myself. Of course, you can mess with pronunciations all that you want; it's your own name for your own race! (Yes, you are free to use these examples; that's what they're here for. ^^)
You could translate a selected word into more than one language, mash the words together, jumble the letters up, and see what you come up with!
demon + akuma (Japanese) ~ Dekuma? I think that that one's already taken . . . Hrm . . .
For even more possibilities, remember that "demon" can also be spelled "daemon," depending largely on dialects and referenced sources.
- Just state that the race to which your character, ally, or whatever belongs is called "Angel," called "Demon," or whatever, but isn't actual angels, demons, et cetera. For instance, those of you who came from Esc know that one person's character developed an aspect by the name of "Demon Summoning." The creatures that were called "Demons" were not actual demons, although many resembled those of folklore and such. They dwelt in and were summoned from a place that was simply known as the "Other Place." It was specifically clarified that the "Other Place" is not Hell, Hades, or whatever else one may have in mind, but simply an alternate realm in which these creatures, which happen to be called "Demons," reside. That same person had another character that summoned "angels" who used many angelic abilities, resembled the most common artist's rendition of angels, et cetera, but were not actually angels; again, they were simply called by that title, but they were alternate beings altogether. Therefore, that person had successfully used the terms of "demons" and "angels" without referring to the respective topics as though they logically were demons and angels.