CREDIT GOES TO DEARHEART
Who is Mary Sue, you ask?
She is a pest, a scourge, a CURSE upon the world of fiction; a dull, cliche character who mostly invades the fan fiction universe (especially in fandoms like Lord of the Rings or Harry Potter), but can also be found in original work, TV shows, books, movies, etc. In fan fiction, she's usually the fangirl's way of inserting herself in the story and pairing herself up with the cutest/hottest character. This infamous evil can be identified by the following traits:
1: She is stunningly/unusually beautiful. Oh she may deny it, claiming to be a "plain Jane" and modestly deflecting any compliments on her looks, but everyone else in the story will be sure to drool all over her. Mary Sue is perfection incarnate - perfect hair, perfect eyes (the kind that "sparkle like jewels" or "pierce into your soul"), perfect skin, perfect figure, gorgeous gowns/snazzy leather warrior outfits, cool-looking weapons, etc...though she may bear a striking resemblance to the author of the story. And she sometimes shares the author's name (that includes middle, last, nicknames, acronyms, etc.) Mary Sue could easily be described as an "author's pet" or a self-insert.
2: She has multiple special abilities or skills (like she can somehow "wield a blade" really well without years of training and experience...unless, of course, she's been trained by the
best since age four). There's nothing she can't do or excel at, everything gets handed to her on a silver platter, and her Speshulness knows no bounds. She could be a shape-shifter with Super Powerful Shiny Magic and a wow-the-crowd singing voice, or a genius computer hacker who can play the flute and violin, has a black belt in karate AND can recite encyclopedia excerpts on a dime, or a half-angel warrior princess who sends the Bad Guys buggering off just by going all "echo-y voice and holy light"...you get the idea.
3: Going hand in hand with the above, she's always being uber-heroic, rescuing random people and saving the day. The fate of Life, The Universe and Everything depends on her. Without her Speshulness to aid the Good Guys in the epic battle, the Bad Guys are sure to win and the world is utterly DOOMED.
4: She's kind and virtuous (unless she's an EVIL Mary Sue) and has no big character flaws of any kind. (And no. Clumsiness and/or a feisty temper do NOT count; they're far too generic and only serve to make her look "cute".) She's usually either a "sweet angel" type, "mysterious woman" type or the "rebellious princess" type.
5: At least one character falls immediately/insanely in love with her, and vice versa (usually the hottest guy. Go figure.)
6: All the good characters in the story just seem naturally drawn to her (especially children and fuzzy animals). She's popular where it matters. EVERYONE likes her. Except the Bad Guys, of course...as well as any unimportant characters who serve no other purpose in the story than to be annoying and/or jealous of her utter perfection.
7: Sometimes there is a prophecy or legend about her. Usually it says something about her Speshulness, an all-important destiny, Twoo Wuv written in the stars, blah blah blah...
8: She is portrayed as being cooler/more mature or more important/powerful than everyone else in the story. Though on the flip side, she could also be portrayed as the poor, defenseless, yet ever courageous damsel-in-distress. Everything's always either about her or her love-interest. She doesn't know the meaning of "teamwork" and she steals the spotlight from everyone else.
9: She often has a tragic/angsty/mysterious past and her love interest has to "heal her wounded heart" or something. If she doesn't have a tragic/angsty/mysterious past, she always finds SOMETHING to angst about...and of course, her Twoo Wuv always rushes to comfort her. Oy.
10: She is so darn perfect and 2-dimensional that it's annoying and boring. She's either so perfect, so weird or so lacking in personality that the readers can't relate to her or grow to truly care about her as her own character. Strip away the beauty, the magic powers, the all-important destiny and the love interest and what do you have underneath? Virtually nothing.
Now that we've met Mary Sue, let me introduce you to her lesser-known cousins! Yeah, that's right...she has relatives.
Meet Gary Stu, the boy version of Mary Sue! He can be all of the above, as well as being the muscular, uberly-hot guy who's always saving the damsels in distress, bravely fighting the Bad Guys, hacking into computers like an expert and diffusing the bomb that's hidden in the wedding cake with the pair of pliers he just happened to have in his pocket.
Last and definitely least, there's Marty Sam. He's closely related to Gary Stu, only he's a lot more sensitive and angsty. Usually he has a dark, super-tragic past (like his alcoholic dad beat him as a kid or something) and he's always throwing melodramatic pity parties about how miserable and wretched he is. He swoons over the beautiful girl of his dreams and goes on and on about how he's such a HORRIBLE MONSTER who's oh-so-unworthy of her. He can be every bit as hawt as Gary Stu, OR he can pull a Hunchback of Dotredame routine, i. e. so ugly that his beautiful love interest can't help but take pity on him and fall in love with him for his "beautiful, broken heart".
So how can you avoid turning your characters into Mary Sues/Gary Stus/Marty Sams?
Well, one thing I've found that leads to a Mary-Sue is getting too attached to your heroine. You might have a brilliant back story for your girl, but does the audience really need to know every detail? You might have designed every ounce of her clothing and styled her hair just right, but is it integral to the plot? Probably not. A lot of the time what makes a Mary-Sue so awful is that so much of the focus is on this character that no one reading the story really cares all that much about her.
Another thing to keep in mind is to avoid having the character be too flawed. If you have a character that has a terminal illness, a bad back, is blind, can't hear, and is wielding a sword in battle and totally kicking butt and saving the day, you've got a problem. It's fine to have a character with one of those "flaws" or traits, but not all of them, and make them act accordingly. Make sure the flaws have something to do with the story and aren't just things you stick in to make her appear less perfect. Too flawed is just as bad as too perfect.
AFTERTHOUGHT: It should be noted that there IS such a thing as a "good" Mary Sue, and that there are a few exceptions where characters can have a lot of the symptoms above and NOT be a Mary Sue (like the elves in Lord of the Rings, for instance). There are many examples of "good" Mary Sues out there, such as Nancy Drew, Sarah Crew from A Little Princess, Buffy the Vampire Slayer and other characters. What makes them different from other Mary Sues is that they are all believable, vibrant, well-rounded, likable characters that people can root for.
Having said that, DON'T get cocky. These kinds of exceptions are rare. Most likely, if you have written a main character into one of your stories who possesses many or all of these qualities, you have created a Mary Sue (or one of her cousins). So get someone to look over your work, lest this infamous pest overtake you!
If you would like to test your character for Mary Sue symptoms, I highly suggest visiting these sites:
The Universal Mary Sue Litmus Test
The Writer's Mary Sue Test: