Posted under General
There is the borrowed_character tag. So if it is a character from somewhere else, but doesn't belong to a copyright, I use original + borrowed character. If this character has its own name, I tag it with that name, when I know it.
Look at list of original characters for examples.
If they have a fairly unique name (first and last name, no other characters by that name) you can just tag them as you would every other character. I.e. arima senne.
If they have a generic or taken name, tag them something like charactername_(creator).
If they don't have a name, you can tag them with a description and their creator, i.e. dark-skinned girl (23).
If they don't really have a creator... I dunno, might be best to ask the forums again.
To be more precise, here's two particular cases I have in mind:
Case 1: An entirely original character. It has a name but doesn't belong to a specific copyright (except its creator's universe, I suppose) and has been depicted by several different artists.
Case 2: An original character that belongs to a certain copyright. It has a name but is not a canon character in that copyright (for example, an original Final Fantasy XIV character/a player character in an MMO, most commonly) and has also been depicted by several different artists.
In both cases, the creators are not artists, and have commissioned, requested, or otherwise obtained art of their character from various artists.
