[You can make a case for any of these characters being “Mary Sues”. From left: Robert Pattinson as Edward from Twilight, Miranda Lawson from Mass Effect 2, Sean Connery as James Bond, and Daisy Ridley as Rey in Star Wars: The Force Awakens. All images via respective Wiki pages and re-used under ‘fair use”]
First, let me begin by defining terms. Or more accurately, letting Wikipedia do it for me:
A Mary Sue (if female) or Marty Stu (if male) is an idealized and seemingly perfect fictional character…
The term “Mary Sue” comes from the name of a character created by Paula Smith in 1973… The story starred Lieutenant Mary Sue (“the youngest Lieutenant in the fleet — only fifteen and a half years old”), and satirized unrealistic characters in Star Trek fan fiction.
“Mary Sue” is now a shorthand for an unrealistically capable character, with no flaws or foibles. It’s the mark of an amateurish writer, too lazy to flesh out their characters.
Naturally, there’s a discussion to be had here about the use of the term’s sexist connotations, and whether the pejorative “Mary Sue” is now used by lazy critics to put down any female protagonist. It’s a very interesting issue, but it’s not the thing about Mary Sues I want to discuss here.
What I want to address is the motivation for creating such characters in the first place. Often, critics assume that the reason is wish-fulfillment; that authors imagine themselves to be these characters, and make them perfect as a result. (Critics usually assume that everyone is as conceited as they are.)
But perfect–or at least, incredibly highly-skilled–characters are actually very tempting for reasons of plot, especially in a science-fiction or fantasy setting. Simply put; when your plot takes place in a big, complicated universe, you want your character to be able to participate in every aspect of that universe.
If I’m writing a sprawling epic with, say, a league of heroic knights who go around fighting dragons, it’s a bit of a letdown if I say “But sadly, Bob the protagonist was an archer who knew nothing about horsemanship or swordsmanship, and so could never be a knight.” By the same token, if Bob is a knight, then it’s a real shame if he can’t be in any archery attacks.
When you’re writing a story, you generally want your protagonist to be able to participate in most of the action. Having them figure out and solve the central conflict makes a better story than: “Bob found out a lot of interesting information about dragons. So he gave it to the experts who handle that sort of thing. 8 months later, he read in the newspaper about how the dragon issue had been solved. ‘Huh,’ he said. ‘So that’s how that all played out.’ The End.”
Now sure, you can have lots of characters with different skill sets, and still have the protagonist be involved in every step. This is relatively easy to do if your setting is the present day or recent past. For example, in a mystery novel, Ted the Brooding Detective With The Dark Past can take the evidence to Jill the Wisecracking Forensics Expert With The Rebellious Streak. (And if they fall in love, then you’ve almost got all the characterization you need.)
But this gets harder to do the more exotic your setting is, because then you have to make up a bunch of skill sets for people This is especially true in science fiction. So, there’s the girl who flies the ships, there’s the guy who fixes the ships, there’s the other guy who fixes the robots, there’s some alien who mines the raw materials for building both the ships and the robots…
It can be done, don’t get me wrong. But it’s tough to do it, and very, very tempting to the novice writer to just say, “We need to get this plot moving! We haven’t got time to meet the guy who waxes the floors. It’s faster to just make the protagonist do it.”