I'm not a lawyer, but I've done my share of reading on topics like this. As I understand things (and as we've discussed a few times around here in different contexts), a "character" has a few parts:
First, you have history, which comes from the character's stories. Those may once have been copyrighted, and that copyright was up for renewal (before the law changed massively). If there wasn't any copyright, or if the copyright wasn't renewed, then the story is in the public domain, which means you may use any or all of the original document in any way you choose.
However, there's a second part, which is more important in some cases: Trademark. You know those Fleischer-produced Superman serials? They're public domain. Does that mean you can adapt them into a Superman comic without DC's permission? No, because DC (Warner Brothers) owns the trademark to Superman, his costume, and his supporting cast. Basically, you can't use as your "mark of trade" (an identifying feature of your work) something that somebody else is using; the courts feel that's unfair, since a customer can't easily determine which product came from which creator.
In almost every case, that means you can use the character, but you can't make the character prominent on the packaging, advertising, or similar trade appearance details.
There may also be patent issues (especially design patents for costumes), but that seems to be extremely rare, and the protection doesn't last long enough to worry about.
So, check the copyrights and renewals on the stories, to ensure what you're using is in the public domain (and never take anybody else's word for it, in case you are sued). Search the Patent and Trademark Office for active trademarks related to the character.
Frantz probably owns an active trademark, if I had to guess, which blocks other people from announcing that a new comic stars Skyman, or using the name in the title (though the Patent and Trademark Office website is going to have a more accurate definition).