Character copyrights and life + X expiration

Asking in the specific context of international copyright law, I would assume that under American law most works with a complicated enough production to have something like this happen would be for-hire.

I've been looking quite a bit into character copyrights and how they're handled differently from work copyrights - things like the Sam Spade/Batmobile Tests, the Popeye scarf case in Japan (and how that seems to have effected the recent Nintendo V Pocketpair suit), and the Only Fools and Horses Dining Experience - If I'm getting this right, character copyrights are considered to be a different entity from the work's copyright, and characters from PD works can't be copyrighted but characters from copyrighted works can be PD? So what happens if someone creates a character for a work in a role other than "author"? Or if there are multiple authors but only one created the characters? Does the character enter PD with its source material or with the solo works of the creator of the character?