In the United States, and with most of the rest of the world, characters are protected by copyright once they are creative enough to be considered a unique work. This means that, once a character is sufficiently original and unique to distinguish it from other characters, they essentially become property of their creators.
However, this doesn’t mean that every aspect of a character can be owned. For example, while the character of Harry Potter can be and is owned by JK Rowling, the idea of a boy wizard or a student of magic is not and can not be. The reason, quite simply, is that such concepts are not considered original enough to be protected.
Characters become protectable through the details that comprise them. For example, writing a story about a boy wizard may not infringe Harry Potter but writing one about a boy wizard that looks like Harry Potter, has his backstory, has similar friends and does similar things probably would be.
Exactly how much it takes to make a character protectable is a gray area, but generally the closer a character is to an already-created one, the more likely it becomes an infringement.
This can be especially interesting when dealing with characters who have some of their stories in the public domain. For example, Sherlock Holmes has been the subject of many lawsuits that have attempted to parse what elements of the character are and are not public domain.
However, for most characters, that is not an issue. Most characters in TV, film and book are not in the public domain, in part or whole. So this raises a different question: What about fan fiction?
Fan fiction, technically, is a copyright infringement. It is taking characters and stories owned by other creators and creating derivative works of them. There is no doubt that it’s a violation of the letter of copyright law.
However, many creators have opted to tolerate or even embrace fan fiction, in particular non-commercial fan fiction.
The reason for this is simple: It doesn’t make much sense to go to war with your fans.
To many creators, fan fiction doesn’t harm their original work and keeps hardcore fans engaged between releases. Because of this, most creators tolerate fan fiction so long as it isn’t presented as an official work and isn’t sold commercially.
Though Amazon’s Kindle Worlds was an attempt to explore legal, commercial fan fiction, the program has been shut down.
The truth is that, as of right now, there is no legitimate market for selling works of fan fiction. However, there are plenty of places to post, share and read non-commercial fan-created works.
If you want to write fan fiction, the best thing to do is to check out the creator of the original works and see what their view on fan fiction is and what rules they have regarding it. Some, such as Anne Rice, don’t allow fan ficiton at all, others, such as Star Trek, have rules regarding fan-created works and others, such as JK Rolwing, openly back fan fiction.
If you follow the rules laid out by the owners, you should be able to enjoy both writing and reading fan fiction without worry.