I feel that I also have something to add to the whole Mary Sue discussion, because it is probable that many of you have encountered two definitions of the same term, and I feel different to them.
First, there is the classic definition of a Mary Sue; and idealized self-insert character that pops out in fanfiction and warps the canon around them in a way that is detrimental to the story, other characters or the internal consistency of the universe.
This character is emblematic of a certain type of bad writing, and it is mostly due to the idea of warping the canon around them, which I feel it is the most clear sign of a Mary Sue.
Then, the other definition, which would be something like: an (usually female) character in any medium whose perceived perfection isn’t appealing to a reader. This is usually why the word Mary Sue is used way too often; it has become some way of saying “I don’t like this character because their skills or personal qualities feels unrealistic to me”. By this definition, any character could be a Mary Sue if it makes you feel this way: Cinderella, Harry Potter, Batman, shopkeeper #1… anyone.
And, let’s be honest. Mary Sue is often a word used to dismiss female protagonist as bad or unrealistic, in a way that isn’t equally applied to male characters. In fact, there is a joke that summarises this pretty well:
“How do you call a male Mary Sue?”
“Protagonist.”