Originally found here. Edited out the dead links, the "universe" thing, and added to the Mary Sue description.
This is an extract from Tara's Fan Fiction FAQ found here.
Who is "Mary Sue"?
"Mary Sue" is the term used for a character who is either:
1. A thinly-veiled fictional version of the author herself
2. An original character who is the protagonist of the story
3. A love interest to a famous fictional character or a few.
Not all "Mary Sues" are cardinal sins. If a good writer commits "Mary Sue", it can still be an entertaining, well-written story despite the above classifications. Examples of this are:
The novel The Romulan Way by Diane Duane
The episode "Lower Decks" of Star Trek: The Next Generation
The character of Engineer Reg Barclay (Dwight Schultz)
However, on the whole, "Mary Sue" stories are written by new writers whose first idea for a story follow a pattern of self-insertion (acting out their personal fantasy vicariously through an original character), and their "Mary Sue" characters suffer from the following characteristics:
1. She's perfect. Literally. Everyone likes her, she can fix the warp core with a bobby pin and a smile regardless of whether or not she's an engineer, she's got an excellent singing voice, and she's psychic too...
2. She's got violet eyes, martial arts training that makes Trinity from The Matrix look like Elmer Fudd, hair down to there, and is usually sleeping with or the daughter of someone we all know and love.
3. She's maverick, headstrong, stubborn, always wins in the end, and always shows "them" how her way is better.
Regarding gender: Mary Sue is not an exclusively female phenomenon. Harry Stu tends to be cocky, maverick, and has all the girls swooning while the captain admires him for his courage, daring, cunning, swashbuckling, computer hacking, and romantic abilities.
Mary Sue characters exist in canon (Wesley Crusher was Gene Roddenberry's Mary Sue, Janeway is Jeri Taylor's, and Seven of Nine is Brannon Braga's,) as well as professional Star Trek novels (Stone from Rock and a Hard Place and Calhoun from New Frontier are Peter David's contributions to the sub-genre). They vary in degree and are not always guilty of Mary Sue behaviour, but have exhibited enough signs to be deserving of the label.
Afraid your original character may be a Mary Sue? Take the The Star Trek: Voyager Mary Sue Litmus Test.
Here are some essays and websites devoted to the subject, that will help you learn how to identify a "Mary Sue" and pointers on how to grow out of that stage of your writing:
Everything and anything you write helps you become a better writer. And yeah, Mary Sues aren't everyone's faves mostly because—on the whole—they tend to be personal fantasies of the author, rather than stories about the characters we know and love. And yeah, a lot of people when they open a VOYAGER story are looking for the VOY characters, and aren't wild about a new character taking up "screen time" (think Seven of Nine last year).
But please please please don't let that dampen your enthusiasm! Keeping writing! Don't be frightened or become discouraged just because you learned something new. And like we've all said, Mary Sues—really good ones, and not the scary kind—can be very good stories and loads of fun. However, try and branch out, and grow past the need for a Mary Sue to tell your stories.
If your original characters continue to take up all your time and creativity, consider creating your own realm for them to play in, and leave the Star Trek realm behind.
If you are so in love with the Trek characters that you can't not write fanfic, then by all means, continue to write fanfic! But try and keep your supporting cast to human scale, rather than operatic, and don't let your additions to the crew eclipse the canonical characters to the extent the show's characters become guest stars in an episode of The Adventures of Mary Sue.
You know what? Even the professionals write Mary Sues sometimes. Look at Peter David's Capt. Mackenzie Calhoun. Calhoun is every single Mary Sue cliché in the book, and while that means some of us won't read Star Trek: New Frontier novels, THOUSANDS of fans love them and keep buying them.
Keep writing. If it's what you're meant to do, then you can't not keep doing it. If it's not what you're meant to do... then someday, hopefully for your sake and ours, you will learn to recognise that and come to terms with it.
Have questions, comments, additions? Drop me a line!
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