When you write you "have a copyright on the entire sample," does that mean you affixed proper notice? This is a "common law," unregistered copyright.
Does it mean that you applied for a registration on that literary and visual work, and got a certificate of registration? This is a registered copyright, and entitles you to "statutory," rather than "actual" damages if there's infringement, as well as your attorney's fees.
The fictional characters themselves, if rendered specific enough, can be protected by a combination of copyright, trademark and unfair competition law. But as my colleague notes, this isn't self-enforcing --you need to do it, or better yet, hire your own IP lawyer for help, to make sure you're acquiring and policing your rights properly.
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I think you are concerned with someone copywriting your work, not trademarking it. If you have a copyright, you need to police your rights. If you find a potential infringer, you must consult an attorney to protect your rights.
As my colleague noted, there might be some confusion here. If someone tried to, for example, use one of your characters as a logo (hence image mark trademark) you can still enforce your copyright rights against that use.
It is very important that you register your material with the US Copyright Office as is required to enforce any infringement claim in court. It will also allow for you to ask the court for statutory damages which are often all that is available in these cases (i.e., real damages are often hard to prove out). If you register your work post infringement these damages will not be allowed.
Obviously, your IP does not police itself and if you suspect infringement you need to enforce against it. Typically, a lawyer can send a Cease & Desist letter for something like this at a starting point.
You may want to discuss this in more detail with a lawyer in private. Most of us here, including myself, offer a free phone consult.
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This does not appear to be an issue of trademark law (although characters can sometimes become trademarks). This is primarily an issue of copyright law. Copyright law protection arises automatically whenever your work is fixed in a tangible medium of expression (such as paper, film, or a computer). Your comic samples are already protected by copyright law even if they have not been registered. Nonetheless, there are many advantages of registration, including most importantly, that if you register within three months of first publication, you are entitled to seek statutory damages and attorneys fees from infringers. This is critically important if you want to have the ability to enforce your copyrights against third parties. Your distribution of samples at a comic convention could be construed to constitute publication and, therefore, it is critical that your register within three months of the convention.
A major question is what should you register? Should you separately register each comic book, each character in the comic book, each story in the comic book, or collections comic books? Remember that you can get separate statutory damages for each work that you register if infringed by a third party. Thus, it is often very worthwhile to separately register each story, or each separate comic book, and each character that appears in your comic books---then when you sue for infringement you may be able to claim several steams of statutory damages. Statutory damages range from no less than $750 for each infringing work to up to $150,000 for each infringing work if the infringement is willful. Thus, the more registered works you have, the more damages awards for which you might qualify.
Copyright remains one of the great bargains---you can copyright most works for $35. But it is important that these copyright applications are filed correctly----if you do this yourself you could make disastrous mistakes. That is why it is advisable to retain intellectual property/copyright counsel to assist you in drafting and filing these applications.
Depending on the circumstances, there may also be merit in filing for trademark protection for your logos and some of the characters. Trademark registration, like copyright registration, is a great bargain compared with other governmental rights (filing fees for patents can be thousands of dollars compared to a few hundred for a trademark application). If you are publishing comic books, then laying the proper copyright and trademark legal foundation is critical, and you should retain and work with IP counsel to review and deal with these and many other issues.
One further thought---I hope that before providing samples of your work at this convention, you retained IP counsel to conduct a clearance analysis---i.e., to determine whether your comic books may violate IP rights (trademarks, copyrights, other) owned by third parties. If you have not conducted such a trademark/copyright clearance, it should be your highest priority. Any writer, publisher, or self-publisher, should retain counsel to conduct such a clearance analysis before releasing a new publication comic book). This is especially important if you self-publisher, because you do not have the benefit of the in-house lawyers on staff at major publishers.
Bottom line--I can tell by your question that you need to retain and consult with IP counsel to keep yourself out of trouble. Like the other attorneys here, I am available for a brief free consultation on any of the above matters.