The Mary Sue or Marty Stu is the bane of fan fiction everywhere. It signifies a character who is a clear stand-in for the writer. It’s a smell of bad writing.
What is it?
The Mary Sue/Marty Stu is a form of wish fulfillment, which is why it’s so common in fan fiction. Most of us have fantasized at some point in our lives what it would be like if we could have been in our favourite story at the time; travelling with the fellowship of the Ring, serving on the USS Enterprise, or going to Hogwarts.
These fantasies inspire a lot of people to start writing. This is – of course – very good. However, when a writer creates a story along the lines of their fantasy, where their main character is a stand-in for themselves, loved by all, with all the cool super powers, pivotal to every plot twist, then we are getting into Mary Sue territory.
When somebody puts themselves in the story, they have a tendency to put the most perfect version of themselves and how they would like to be in the story. A character in a story needs to be well-rounded, and they can excel at some things, but not all things. They need to have flaws as well as virtues. In other words, they should be realistic.
Note that this is a lot of grey in this area. What one person might call a Mary Sue, another might call a fun protagonist. Stories are about exceptional people, so when are they no longer exceptional, but just unrealistic?
As I said, fan fiction is rife with Mary Sues and Marty Stus. Just Google ‘Harry Potter fanfiction‘ or ‘Twilight fan fiction‘ and you’ll be swimming in examples in no time. Now, I am not saying all fan fiction is bad, or that all fan fiction contains Mary sues or Marty Stus. Fifty Shades of Grey is a Twilight fan fiction, for instance, and I wouldn’t call that a Mary Sue story.
Interestingly, in Twilight itself, Bella can be characterised as a Mary Sue. She has no real personality, but all the boys fall head-over-heels in love with her. She is considered smart and beautiful, but she displays no real smarts and is described as plain.
It has been said that Rey from the Force Awakens is a Mary Sue. I tend to disagree. She has a lot of skills, like mechanic, piloting, and more, but there are back-story reasons why she has those. Also, the fact that a character is exceptional does not make them a Mary Sue or Marty Stu. The fact that they are exceptional for no reason,without flaws, and have no arc makes them a Mary Sue or Marty Stu. And that is not the case with Rey.
How to fix it
The most important thing is to take a step back. Being invested in a protagonist means you cannot judge them objectively, but you can try.
Take a good look at the character: are they always in the limelight? Are they never wrong? Does everybody love them? Can they solve any problems thrown at them? If the answer to some of these questions is yes, then the character might be a Mary Sue or Marty Stu.
To fix it, give them a flaw. Make them arrogant, or obnoxious, or elitist, or mentally unstable. Those flaws need to play a role in the story. Then make sure not everybody loves them, especially if they are very good at things.
If they are still too over-the-top, you could split them up into multiple people. Imagine if Harry Potter and Hermione had been one character who was both the destined saviour of wizard-kind, and incredibly smart. He would have been a bit of an ass. But by having those good qualities spread out over multiple people, the story was more balanced. And it allowed for more characterisation and subplots.
Mary Sues and Marty Stus are not the end of a story, but they do require work to fix.