The Sue couple are a negative feature in writing used to describe a poorly crafted character (where Mary Sue is the female character and Gary Sue the male – obviously). I just say Sue because it’s easier. Anyway, people are a little divided over the true definition of a Sue. I think the main reason there’s so much disagreement is because people are trying to identify specific traits that are representative of a Sue. It doesn’t work that way because writing isn’t so flat and two-dimensional that you can just say a group of traits is bad. Before I tell you my definition (which I think is – if not better – then at least more encompassing), let’s get some history down.
The name “Mary Sue” comes from the 1974 Star Trekfanfic “A Trekkie’s Tale”. Originally written as a parody of the standard Self-Insert Fic of the time (as opposed to any particular traits), the name was quickly adopted by the Star Trek fanfiction community. Its original meaning mostly held that it was an Always FemaleAuthor Avatar, regardless of character role or perceived quality. Often, the characters would get in a relationship with either Kirk or Spock, turn out to have a familial bond with a crew member, be a Half-Human Hybrid masquerading as a human, and die in a graceful, beautiful way to reinforce that the character was Too Good For This Sinful Earth.
From that passage we can identify two things, a Sue is too perfect and is an “author avatar”, which basically means a character through which the author inserts him/herself into the story (most commonly in fanfiction). As a result, Sues have been categorised as having traits such as “too strong, too beautiful, too whatever”, which can collectively be considered as being too perfect. Others argue that Sues are when the author projects themselves into their story (refer to Twilight rant). By projecting themselves, I mean a writer who creates a character representing certain aspects of his/her own personality, and makes good things happen to that character to compensate for a feeling of inadequacy in real life.
These are very subjective measures though, hence the debate over the meaning of a Sue. The problem is that sometimes, your character may have traits that would normally be considered a Sue trait, but is not when placed in context of your story. Here’s how you can prove what I just said – take your most original character and do this test: http://www.springhole.net/writing/marysue.htm. You’ll find that you get a far higher Sue score than you’d think (try and be objective here and admit if you’re overpowering your main character).
Here’s a clearer explanation from me (the Sue test is just for fun – I make a lot more sense than it does). Just because your character has some traits that liken to Sue traits does not make it a Sue – it all depends on your story. For example, if you’re writing about a pickpocket-turned-hitman from the slums of a medieval setting, having him/her being ridiculously good looking, charming and the most sought after sexual target in the story is sort of pushing it. Not only is that ignoring the setting (hygiene was no where near current levels in medieval times), but it’s ignoring the fact that the character is from the slums. Even if you made the setting a modern day society, things get questionable when your character is far too attractive. If you throw in a few too many good traits, you’ll have a Sue. Next example: what if you’re writing about the political side of heaven, where angels debate whether they should interfere with the devil’s work on Earth or let things run its course? Well, it makes sense if your character is good looking because he/she is a freaking angel – they’re all meant to be good looking.
As for Author Avatars, well, it’s one thing to take out your insecurities on a character and it’s another to base your characters off real human emotions. The distinction can be blurry to a reader, but I think the latter makes for a much stronger and more believable character. They key here is to include both good and bad traits – perfection is your enemy (unless you’re writing about angels)!
So, what is a Sue then? My definition is: A character that is unrealistic within the reality and mechanics of your fictional world. Unrealistic encompasses both perfection and a lack of emotional range. Realistic characters will have strengths and weaknesses. As a writer, you need to make your story engaging, and it isn’t when your character is unbelievable. This definition does not limit your ability to use supernatural abilities, magic, sci-fi technology or anything else “unbelievable” in modern day terms because these things are explainable within the reality and mechanics of your world. If you write a sci-fi, then technology is explainable. If you write about magicians, then magic is explainable. They are not unrealistic in your world. But if you write about cavemen, except your main character has a giant robot mecha suit and uses it to conquer the stone age, then yes, you’ve gone too far.