We'll never know, because I don't write fan fiction.
So why am I writing about fan fiction? Because some of you do write it, and you want to know if it's legal.
Is it? Sometimes yes, sometimes no. Don't you just love lawyers who won't give you a straight answer?
Trademark and copyright are the two main areas of law that affect fan fiction.
Trademark is easy. Simply disclaim any connection to or endorsement by the trademark owner, and you're fine.
Copyright is harder.
Copyright protects expression, but it does not protect ideas or names. If you write about a flying superhero dressed in white who saves sailors from the forces of evil, all you've taken from Superman is an idea, and you haven't infringed the copyright. If you write about a boy named Harry Potter who lives in an orphanage in London in the 1800s and stands up to the matron by asking for more gruel, all you've taken from J.K. Rowling is a name, so you haven't infringed her copyright. (If Charles Dickens' descendants owned a copyright in Oliver Twist, that would be a different matter. But they don't.)
On the other hand, if you take a well-developed character like Harry Potter, put him in his familiar setting, and have him use his wizardry skills to fight evil, you have probably taken some of Rowling's written expression as well as her hero's name. So you may be infringing her copyright.
If imitation is the sincerest form of flattery, J.D. Salinger should have been thrilled when a Swedish writer wrote a "sequel" to Catcher in the Rye. But Salinger sued instead. According to the District Court judge, the Swedish writer aged Holden Caulfield sixty years and added Salinger himself as a character but made very few changes to the original story.* Although the case was in an early stage, the judge indicated that Salinger had a good chance of succeeding on his copyright claim.
Even if you don't use the character's name, you could violate someone's copyright by making the character easily identifiable. I assume you recognized Superman and Harry Potter in the first paragraph. If I were to complete that story and try to publish it somewhere, I could be in trouble. (This post is okay because the way I used those characters here is a "transformative fair use." But fair use is beyond the scope of this article.)
One aside: true parodies don't infringe copyrights, but most fan fiction doesn't qualify as parody. And the courts won't assume your work is a parody just because you say so.
So here are some general guidelines for writing fan fiction about well-developed characters.
(1) If you are writing purely as a creative exercise and are the only person who sees the story, you should be fine.
(2) If you want to post on your blog or a fan fiction website, check the Internet to see how the copyright owner (usually the author) feels about fan fiction. Some are flattered and see it as a good source of publicity, while others oppose it.
(3) But if you are doing it for profit, beware. The only safe course is to get the copyright owner's express permission.
Kathryn Page Camp
* Salinger v. Colting, 641 F.Supp.2d 250 (S.D.N.Y. 2009). A subsequent appeals court decision vacated the judge's order for a preliminary injunction but agreed that the J.D. Salinger Literary Trust (which inherited the copyright after Salinger's death) was likely to prevail on the copyright issue at a later stage in the proceedings. 607 F.3d 68 (2nd Cir. 2010)