Yes, Virginia, you ARE a Mary Sue
Some people are so convinced that the character they are writing is not a Mary Sue, that even if she wins the hearts of every male character in the series and then goes on to save the world, they still won't believe. The reverse side of this, which I see as far more insidious, is that there are those who believe that every single original female character written into a piece of fanfiction is a Mary Sue. And of course, we have those characters who are supposedly canon - only, the author has Sueified them.
So let's talk about how to identify and tag a Mary Sue in the Wild.
First of all, there are websites that will help you identify Mary Sues - there's a rather famous one called the Mary Sue Litmus Test, but I don't trust it. As an experiment, I took Harry Potter - a rather Sue-ish character, truth be told - and contrasted him with Aragorn, whose only claim to Suedom is his un-claimable throne and his skill with a sword. They are both apparently Sues, according to the test!
Before we get started, let me say that for some of the examples below, I'm going to refer to characters that are NOT necessarily Mary Sues or Gary Stus - but perhaps they have a trait or two of the said Sues. I am not calling Ranma, Anne, Harry, or Hermione Mary Sues!
Much.
So - here are a few of my own guidelines - the stuff that inevitably sets off a warning bell if I see it...
1) The character's name is odd for their culture or area.
This includes names that are especially frilly, like Schisandra, River, and Melisande, as well as names that seem purposely misspelled, like Ambyr, and names that seem out-of-place, like Yoiko the Canadian. People should also not be named after objects: no 'River' or 'Moonbeam' or 'Blade'.
2) The character has a dark and / or tragic past.
Characters like Harry himself fit this description, and so does Ranma. Hey, what's life without a juicy past, after all? But when the characters' past is so darkly tragic that the main character uses it to excuse extreme actions, that's when we stumble into the realm of a Sue - or, more often in this case, a Stu. Other characters weep for the Stu, while he remains Strong and Manly - and also a little crazy - in the face of his angst Traumatic Adversity. These Stus often live for revenge, and unconvincingly claim not to care for anything else. Overdrama abounds.
3) The character belongs to a marginalized sociocultural group.
Oh, this one has to be my all-time favorite. Just like No. 2, this one has an all-important addendum. The real key here is not only that the character belong to such a group - but that the reader cannot possibly personally dislike the sociocultural group in question. A perfect example is Rowling's Hermione Granger. How can the average reader of a Harry Potter novel possibly care that Hermione's parents are not part of the culture of magic? We don't. But the rest of the school knows and cares - and many people avoid Hermione or tease her because of it! We sympathize without ever really understanding how someone could dislike Hermione for that reason. It's a cheap trick, and everybody uses it.
4) She is adored by the majority, and hated vehemently by the minority.
He is hated by the majority, and passionately adored by the minority.
Does every-bloody-body love your character - if not at first sight, then eventually? Does he or she have fans? Somewhat more importantly, is there a small group of folks who want to spit in her face? Is the reason they hate her because of the attention she receives? Finally, the kicker: if someone doesn't like this character or shows them disrespect, does that automatically mean they are evil? If so, you'd darn well better make a character who is intrinsically good and also hates their guts - or at least is indifferent to their obviously considerable charm. These tend to be Mary Sues.
The reverse is equally suspect. If your character fights against the hatred of the entire universe save a loyal few, and is constantly misunderstood and cast down, he'll probably prove everybody wrong in the end. But isn't it romantic, meanwhile, how oppressed he is? Yuh huh. These tend to be Gary Stus.
5) The established universe warps around the character.
The rules are just different for your character. Your character can say and do things that are too forward, too blunt, too arrogant or too intimate, but without any of the reprecussions that other characters experience. While around your character, smart characters get dumber, suave characters are suddenly tongue-tied, and savvy magic-wielders and sword-fighters stumble in battle. This, you may have guessed already, is to make your character look Cooler. Woot! See how making other characters lame makes your lame character seem less lame?
My favorite example of this little trick is the demonization of a traditionally good character in order to redeem a traditionally evil or mischevious character. Usually one or several characters who were basically good in canon are chosen to be scapegoats. They are made Evil by the fanfiction author, who then proceeds to redeem initially evil characters, who are apparently less evil now by sheer comparison.
Another thing that falls under the Universe-warp is when the rules you've set up for your world, whether magical, physical, or spiritual, apply to everyone but your character. An Avada Kedavra kills anyone, wizard or Muggle - except for Harry Potter. Animagi can only turn into one, non-magical animal - except for your original character, who turns into a centaur, a House Elf, and a pink, fluffy unicorn.
6) The character has Special Skillz.
This falls only slightly out of the realm of Universe-warping. Here, your character possesses skills that are admittedly possible for him or her to possess, but highly improbable. The ability to speak a dozen languages, even if allowable by the rules of your universe, just doesn't seem likely. Mary Sues often sing like a nightingale, play an instrument, dance, sketch, or paint, which would be fine except that she doesn't just dabble - she's likely a master of her craft, despite being all of fourteen years old. If she exists in a world of magic, she's ahead of her peers by leaps and bounds, in power, control, and skill. If she fights, she handles six different weapons with the ease of a battle-hardened warrior. Perhaps she has ESP. The key here is that she has skills that her peers just don't have, and she doesn't seem to have attained them through hard work - just through pure talent.
Would that we could all be so lucky.
7) The character's parents are dead.
Just like it says. Extra Sue points if the folks who ended up raising him or her were abusive and mean. More extra points if he or she is subsequently whisked away from them.
8) Physical attributes are described at the drop of a hat, and at length.
The character's physical attributes are somehow important to the plot.
The character's actions are described using purple prose.
Are your character's hair or eyes described as the colour of an element or something in nature, such as stormclouds or jade? (Or worse, chocolate?) Does your character have a scar in an attractive place? Does your character have no physical flaws?
Often, an author will attempt to de-Sueify a character by giving her a single flaw, which only sets off her awesomeness. In these cases, the flaw is only seen as a flaw by the character herself - other characters and indeed, the readers themselves, either deem the flaw of little note; or they actually see the flaw as desirable. Anne of Green Gables's abhorred red hair is a good example of this - she despises the colour of her hair, but none of the other characters - or the readers - care. In fact, her hair - supposedly a flaw - only made her seem more exotic and different from the other characters.
(A note to beginning authors: the reader doesn't care:
a) How thin your character is
b) How pale or tan their skin is
c) What colour skirt they're wearing, what earrings they chose today, and the fact that they're wearing their favorite clogs
d) Exactly what makeup they put on
We care that they wear makeup, because it says something about the character. But giving us a play-by-play of the character's ensemble is a dead giveaway that you consider the characters' looks to be essential to the workings of your plot, such as it is. Nearly always a bad sign.)
Additionally, if your character weeps instead of cries, sobs or sniffles; if she glides instead of walks; if she whispers and mumbles and trills instead of speaks - try and tether her back to earth with the rest of us.
9) The character is sort of like you - I mean, kind of like you! But not entirely!
Oh, abandon hope all ye who enter here. I'm so dreadfully sorry, but if you base a character on you, he will suck. Do you remember Wesley Crusher from ST:tNG? Well, maybe not, but he sucked. And do you know why? Because Gene Wesley Roddenberry based the boy on himself.
Of course you can base characters on one attribute of yourself. Your stage fright. Or your impulsive nature. Or your crippling shyness. But add even one more trait that reminds you of, well, you, and it's a slippery slope, my friend. Before you know it, you've equated that character with yourself, and he is lost.
10) The character is a loner who flouts authority, and needs no one.
Your character is just so reeking with awesomeness that he doesn't keep friends, longtime lovers, or family around. If family is ever mentioned, they died - tragically. He feels really guilty about this, though there was actually nothing he could have done to prevent it. He has acquaintances and allies. Friends? He don't need no stinking friends, and no one tells him what to do!
11) The character has a 'look' that is either stereotypical, or completely unique to him or her.
He wears a trenchcoat in modern-day. Twenty years ago, the jacket was made of leather, but jackets always seem to be involved. He smokes. He carries a gun. And knives. Despite the fact that since he is written in the HP Universe, he should have a wand! Despite the fact that since he is written in the Ranmaverse, guns are literally never shown in the anime or manga! He just has one, damn it! 'Cause they're cool, all right?
The feminine version? Think 'magical girl'.
12) The character is supernaturally romantically desirable.
Your character leaves broken hearts wherever he or she roams. Everyone either loves this character or is jealous of him or her. If an original character, she often causes the main male character to fall for her by walking past him, blinking at him, answering a question in class, or breathing.
13) If the character were real, she'd be mentally ill.
Stop for a moment and consider your character very, very carefully. If your story were real life, how would your character be viewed? Would they be seen as having delusions of grandeur? Schizophrenic? Abusive? Bipolar? Clinically depressed or suicidal? Manic? Bewilderingly, beginning authors often mistake insanity for outrageous coolness, or the actions of the insane for 'drama'. If your character tries to kill herself, this is not 'cool', or 'moving', at least not the way it's handled most often. The only time I've seen it done well in recent memory is here.
14) The character's skill set expands dramatically over the course of the tale.
As the story winds along, your character picks up magical objects, new skills, additional martial arts techniques, an inordinately powerful new wand, realizes he is part-magical creature, collects all the Dragonballs, and amasses the largest collection of Pokemon in history.
This, like several of the other points listed here, has an all-important addendum: there must be little to no plot in the story aside from the quest to become more powerful. There are often perfectly valid reasons for Ranma to train in a new technique, or for Harry to garner more power; but before very long, even these original, workable plot-points disappear into the maw of your character's mad desire for more. Any faint, fledgling plot devices wither in the face of it.
Mary and Gary in general are somewhat sad little figures when all is said and done, but I always find this the saddest and scariest of our fifteen possible Sue- and Stu points. First of all, the power garnered by your Stu is all-too-often used to debase the other canon characters, to prove Gary's supremacy over all his surroundings. Women especially are often debased by power-up!Gary; and enemies aren't dissuaded, they're destroyed. Second, the power is never quite enough for your character - he spends the story constantly shooting for more. Therefore, these tales are either perpetual 'WIP's (works-in-progress); or they end quite abruptly without a final confrontation or anything that could be seen as a conclusion. In literature as in life: there is no real conclusion to a quest for power in any of its forms.
Power-up heroes can be fun; just make sure that the enemy is very powerful, too, and continues to get more clever and more powerful as your hero does, both ensuring that he doesn't always win, and giving him a concrete reason for desiring more power so passionately. Still - there is no one point here that is quite so damning as this one: if this describes your character, he is almost certainly a Gary Stu; and no one is enjoying his rise to stardom but you.
15) The character dies.
Yup. Points if they die heroically. Extra points if they do so to save their beloved. Double points if they do so to save a race, planet, or empire. Go, Spock! And if they come back from the dead, afterwards? Well, that qualifies them for super!Sue-dom. Go, Xena!
Use these laws with common sense. Fulfilling one of them doesn't make your character a Mary Sue or Gary Stu. Even fulfilling two or three is all right, so long as you're sure to give your character plenty of flaws to offset their good points. I think it's safe to say that four or more is a reason to re-evaluate your character, if not dismiss him or her completely.
That's it for now, folks! Refuse the Mary Sue and Gary Stu! Deny them! Do not write them! Do not read them! ;)
-K