(It would appear that this article has all my tags that start with "f." Funny.)
Recently, my sister has been discussing this subject on the Comic Dish forums. Or rather, "What makes a Mary Sue?"
In case you're not familiar with the terminology, a "Mary Sue" is a character, usually (but not invariably) associated with fanfiction, who generally plays out the fantasies of the author, is excessively powerful, and loved/hated intensely by all, presented as the greatest person ever, etc.
Lots of people have their own ideas on what makes a Mary Sue. (Hereafter referred to simply as a Sue for convenience. Not that I have anything against women named Sue; I have a very nice aunt named Sue. It's just the shortest and most-agreed upon abbreviation of the term.)
I tend to think that the identifying features of a Sue aren't quantitative, but qualitative. Thus, "litmus tests" don't work very well, in and of themselves. They count characteristics, and if someone has enough of these (some of these tests are actually made so that real people can theoretically be subjected to them), that person's a Sue, no ifs, ands, or buts.
The qualities of a Sue, in my own mind, are as follows.
1. The author clearly thinks the character is very awesome.
2. The reader doesn't.
Part of the process of "Sue creation" is telegraphing. When an author, particularly an inexperienced one, writes, a bias appears. Consciously or no, the bias tips off the reader that the author thinks the character is really great. Thus, the reader hopes to see the character be awesome, and when the character proves not to be so much, the reader says "Sue."
Generally, "Suisms" are the result of sloppy and inexperienced, occasionally lazy writing. The author tries too hard to make the reader like the character, and isn't that good at the methodologies of making the character likeable. (For instance, many Sues are infamously morally ambiguous, or sometimes downright evil, yet they are treated as if they're wonderful people.)
Another common characteristic of Sues listed on litmus tests is "unusualness." If a character isn't unusual, though, what's the point?
As I sometimes do, I'll use an example from my own writing. No fiction involving the character herself is actually written yet, but she's an excellent example of someone who would score extremely high on the typical litmus test for reasons completely beyond her own control. (I'm using her partly because I have a ready and convenient reference for her, and her existence has been established on the ol' internets.)
(I would embed the picture from DeviantArt, but you'll just have to be satisfied with clicking a link because embedding isn't enabled.)
Lheabi here is one of the wildest characters, certainly in terms of appearance, that I've ever come up with.
And, of course, she has an unusual name, one of the first things that Sue litmus tests attack. I'll be frank: I don't remember where it came from. I think I keyboard-mashed it into existence, but I could be wrong. Whether or not you can consider it unusual and meaningful depends on whether or not you consider random keyboard-mashing to have meaning.
She's attractive, which my sister (the artist) wags on a bit in her description. This certainly would score her points on any litmus test. I'll let you in on a secret which isn't technically spoilery: She's had a bit of work done. (Technically speaking, that's unusual for the setting in general, but not the subset of it which she comes from. Well, it is unusual for that subset, but it's complex. The rules weren't written to suit her, though; she evolved from the rules.)
She's ethnically mixed (in the fantasy setting sense). She's a wild mix of elf (oh, no, not a part-elf! 300 litmus test points!), y'cag (a race I made up, with blue or red skin and horns), bentang (another race I invented that is basically just multicolored humans, and the probable source of her hair and at least part of her skin coloration), and zzzyxan (guaranteed to be alphabetically last every time-known for being very strong and pseudo-albinos [and yes, I made them up-I make up a lot]). Oh, no, four races that aren't typical-stock human? (Fantasy races follow very different rules, obviously. On a scientific level, it'd be more correct to call them human subspecies, which is why I make the "typical-stock human" distinction.) Well, for me personally, it isn't much of a stretch. She's got four grandparents, doesn't she? The peculiar society she comes from encourages equality among the races (including interbreeding), so it isn't that strange. She retains some of the characteristics of these various races (her ears aren't as pointed as most elves, and not mobile, as most of my sister's elf ears are; she is probably older than she appears, but less so than if she was a full-blooded elf [my elves from this particular setting have very long lifespans]; she's quite strong, but proportionately less so than a full-blooded zzzyxan; and her horns are unusually big considering her low proportion of y'cag heritage, but she may have tweaked that, and as I said she's older than she looks [y'cag horns grow continuously throughout life]), but in limited measures.
She also has characteristics unusual even for what she is. (You may have noted that not one of the races listed had "six arms" or extra eyes [it's a bit subtle, as it doesn't look "normal," but there's one on her forehead] as a characteristic. She may be a fantasy character, but she's still a tetrapod, for crying out loud!) Well, this is fully explicable. The peculiar society of which she is a part has much in the way of advanced "technology," the most sophisticated of which are their surgical and related techniques. This includes extensions of the skeleton, added muscles, and other organs. It is not at all uncommon for these people to have many additional eyes, limbs, and even internal organs. (In fact, Lheabi has a specialized artificial internal organ that actually will do the "work" for this kind of thing. It's how-er, never mind, getting a bit deep in the story there. Let's just say that she probably was asking for enough modifications that the guys who were doing the work figured it'd be easier to let her have one of those so she'd leave them alone.) And of course, technically, it's how most of the other "work" Lheabi's had done was achieved.
That maid outfit? Totally a random fancy of my sister's. Just so you know. (She does that. In fact, the entire picture was a random fancy of hers that turned out particularly nicely.)
Anyway, personality is important, too. Generally speaking, Lheabi is relatively shy and quiet. (No, seriously.) She only "opens up" around her friends, most of whom originally are members of a sort of counterculture group in the society she was born into. Rebels without a cause-even hippies, you might say. What do these people do together? Um, things their society wouldn't approve of. (Actually, none of it would be considered illegal, or even particularly questionable, in the United States by most parties.) She is extremely affectionate, and doesn't generally like violence. She will, on the other hand, defend herself vigorously if she thinks she stands a chance. She's also insecure, which is part of which drove her to... um, have as much work done as she did.
But here's the thing. As much thought as I've put into the character (or into the factors that resulted in me creating her on a whim), and as much as I like her, she's not actually that important a character. Other characters' reactions to her are determined by how they react to an attractive (albeit strange-looking) woman, not this quadri-racial, six-armed, devil-horned, and slightly loopy mass of vaguely Suishness. She doesn't have a grand mission, a great destiny, or unique or special powers. And if she ever does anything totally crazy, the other characters will certainly tell her so. (Except her friend Gorce. But that's another story.)
Is she Sue? I don't think so. Some readers might. It'll be up to how well I pull off writing her (when I get around to it) that will really determine how people feel about her.