Characters aren't copyrighted or public domain, their stories are.
As much money as people pay for the "rights" to a character, they have no standing if another person publishes a book derived from content that first came from a source that's in the public domain. Oh, plus there are trademark considerations in advertising.
Let's take Superman as an example. Superman is, lock, stock, and barrel, owned by DC, right? Pretty much, but the Fleischer cartoons have fallen into the public domain. So you can create your own Superman based on those cartoons, plus or minus any ideas that were created earlier and are still protected by copyright, like being sent from a destroyed Krypton and posing as Clark Kent.
You also can't prominently display Superman's insignia or name in the "product dress" (covers and advertisements), because that would violate DC's trademark. When people see "Superman," they assume it's coming from DC, and you're not allowed to steal that goodwill.
As to your specific situation and characters, you'll have to do the copyright research yourself and you'll probably want to check with a Intellectual Property attorney (after gathering the research) to make sure you're safe.