Explaining Tropes: Mary Sue/Gary Stu

A Mary/Gary Sue is a difficult term to define, and the term is frequently misused. The line between interesting and a sue can be a matter of opinion, something writers are painfully aware of.

There are traits that commonly show up, but are neither the immediate indicator of a sue, nor universal with them.

For example:

• Being beautiful
• Having a hard life (usually to a very exaggerated degree)
• Their look may be unrealistic considering their habits (for example, a healthy-looking anorexic)
• Characters look like a celebrity, or the author
• Unnatural hair or eye color
• A hybrid of more than two species (like half vampire, half werewolf, half human)
• Weaknesses or faults like 'clumsy' or being 'passionate.' 

There's two major indicators of a Mary Sue: How the author treats them, and how the other characters treat them. The author might be incredibly close to or defensive of their character. Any criticism is met with hostility, as if you were insulting the author themselves. Sharing traits with the author, like looks or even their name, is a nearly universal indicator. 

Inside the universe, there are two things to consider: Is the character exempt to the rules of the universe? (For example, Bella getting pregnant by Edward, despite it being explicitly explained that vampires can't reproduce.) Do characters fall over themselves for the character, despite the character's actual quality, or their original temperament? (Like Bella's circle of friends, and Edward's being taken with her despite being a loner his entire life, and how the vampire leaders are willing to break the rules for her.) 

With Sues/Stus there is usually a lack of balance within the character. They are treated as the most awesomest thing that ever existed. They are gorgeous, strong, and loved by everyone. But weaknesses? Faults? Bah! Sometimes writers will attempt to throw us a bone by giving a fault that is considered very minor, but it can't even  begin to balance out their virtues. 

Finally, Sues/Stus will never see consequences for their actions, and if they do, everyone is being 'incredibly unfair' and we are clearly meant to pity the character. 

I want to stress that Sues/Stus are not characters you dislike. For example, Ginny Weasley is liked by most of the fandom, but it cannot be denied she is a sue; she is great at magic, she escapes the second book completely unscathed and with nobody even remotely upset with her, and she ends up with Harry Potter. It is all about how they are treated by the author, and the other characters in the work.