Three, maybe four issues. Apart from the rudeness of "oh, I'm going to discuss my project in public, but don't want to hear anything negative about it because I'm sure you guys are whiny." (Personally, I was going to point out that "Watchmen" and its ilk was already a cliche when it was published, and the millions of copycats since haven't made it better, but hey...)
First, there's no legal concept as a "character," so they're neither copyright nor public domain as such. The works they appear in are, and for a comic character, that could be dozens or hundreds of works. If you derive your work from any copyrighted work (no matter the characters, ignoring various Fair Use ideas), that's copyright infringement. Also, with a stated target of being in a particular class of works, you might want to make sure you're not infringing on any of them, either.
Second, "I heard on the Internet" isn't worth anything in court, and giving legal advice without a license is also illegal in a lot of places for that reason. If you actually need to know the copyright status, don't trust us or anybody else. Do the research yourself and keep records of where you looked and what you saw. Careful, documented research is a requirement. We've got threads about that, describing what's involved, floating around the can/can't upload material.
Third, copyright law provides no penalty for being wrong, if you're a plaintiff. Well, that's not entirely true. If the case goes to court and you defend yourself successfully, the plaintiff pays your legal fees (and vice versa if you lose). However, that requires going to court for however long they drag out the case. So there's never a completely "safe" state, just a judgement of "safe enough" you'll have to make for yourself. There is actually a growing industry of people who threaten to sue, in hopes that you'll settle out of court and give them a few thousand dollars, rather than spend money and time in court. So every business venture takes its chances.
Last, using a character isn't just about copyright. It's also about trademarks and, potentially, design patents. With the "gold rush" of public domain characters, over the last few years, it's entirely possible that someone has registered a character design, name, or terminology or has been using it long enough that you'd be infringing there.
If it concerns you, I'd recommend getting in touch with an Intellectual Property Lawyer in your area. What may just be a couple hundred bucks will get you actual advice, more reliable protection against legal problems, and knowledge of how cases have gone in your district.
