Let's talk about Mary Sue.
We've all met her. She's the violet-eyed, crimson-haired, secret daughter of Amadala and Obi Wan, sent to be raised on the hidden planet where the last Jedi ran to escape the war, and she has just emerged back into the universe with her spinning light saber batons to save her half-brother Luke from falling to the Dark Side. She's the missing Winchester sister with the two magic guns, one for shooting angels, one for shooting demons, who just fought her way out of Purgatory to rejoin her family. She's smarter than you, she's prettier than you, she's more competent than you, her milkshake brings all the boys to the yard, and the odds are good that she doesn't even notice, because she's just existing in her happy little cloud cuckoo land of sunshine and zombie puppies. Mary Sue, like mistletoe, is a parasitic growth, only she grows on stories, and not on trees.
Mary Sue is misunderstood. She's a cuckoo egg left in a starling's nest, hatching into something big and bright and demanding that doesn't belong where it is. In her own story, she would be something else altogether: she would be a protagonist, she would be the biggest, brightest thing in the room because that's what a protagonist is. But because she's trying to be a starling instead of a cuckoo, she's out of place. She doesn't work. That doesn't make her welcome where she is...but it does mean that maybe all Mary really needs to do is fly away home.
Meeting Mary Sue.
In fanfiction, Mary Sue was used specifically for an original character, often closely resembling an idealized version of the writer, who was inserted into a world and caused the world turn upside down and reconfigure itself around her center...The problem with using this term outside of fanfiction is simple: the world of a novel has always configured around main characters. They are at its center and, often, they are the best at stuff.
—Holly Black, Ladies Ladies Ladies.
Mary Sue, like mistletoe, like cuckoos, has a natural habitat, and that habitat is fan fiction. She is the character who steps in and warps the story beyond all recognition.
Can she exist in original fiction? Yes, but it's harder. Usually, she'll be the minor character who somehow winds up rising from spear-carrier to scene-stealer to magical-perfect-solution-to-everything. Can a central character be unlikeably perfect, never challenged by anything, and all too ready to solve every situation with a wave of her perfect hand and a flick of her perfect hair? Yes, but that isn't the same thing as being a Mary Sue.
Not all Mary Sues are author self-insert, although the majority will have some aspects of self-insertion. Really, what makes Mary Sue Mary Sue is this:
Mary Sue breaks the story.
Mary Sue arrives on the scene and everyone loves her, instantly and without question. Mary Sue is adorably insecure, but only so she can be even more perfect. Mary Sue has a unicorn in a science fiction universe, and a robot butler in a fantasy universe. Mary Sue either gets the hero, or heroically arranges for him to be with the heroine, because she's too good and nice and wonderful to stand in the way of destiny. Mary Sue changes the game...and she is able to do so because the game isn't hers. If Mary Sue owns the game, then her name changes, and she gets to be something other than a concept.
She gets to be a person.
Eves and Apples.
When I read reviews, I see the term Mary-Sue used to mean:
1) A female character who is too perfect
2) A female character who kicks too much butt
3) A female character who gets her way too easily
4) A female character who is too powerful
5) A female character who has too many flaws
6) A female character who has the wrong flaws
7) A female character who has no flaws
8) A female character who is annoying or obnoxious
9) A female character who is one dimensional or badly written
10) A female character who is too passive or boring
—Zoë Marriott, You Can Stuff Your Mary Sue Where the Sun Don't Shine.
The definitions of Mary Sue are often contradictory, as are the definitions of her male counterpart, Gary Stu. That being said, I have seen many, many female protagonists accused of Mary Sue-ism, but have very rarely seen the opposite accusation leveled at male protagonists, even when the weight of the definition seems to point much more firmly at the males in the situation. Harry Potter is the son of two incredibly beloved, talented, respected wizards; he's never been exposed to the wizard world before the start of the series, yet is instantly one of the most skilled Seekers the Quiddich Team has ever seen; all his flaws turn out to be advantages; everyone loves him, or is instantly branded a villain for ever and ever and ever. Hermione Granger has worked hard for everything she has. She's the smartest girl in Gryffindor, but that's about it; she isn't naturally incredibly magically talented, or handed all her advantages for nothing. Yet I see her accused of Mary Sue-ism way more often than I see him accused of Gary Stu-ism.
Half the time, "Mary Sue" seems to mean "female character." And that doesn't work for me, for a lot of reasons, including "I write female characters who aren't Mary Sues," and also, "if all women are Mary Sues, why does my hair get frizzy when it rains?" (I would totally be willing to be a Mary Sue if it meant my hair was always perfect and I could go to sleep wearing eyeliner without waking up the next morning looking like a raccoon.) Male characters get to be competent or obnoxious, skilled or clumsy, intelligent or ignorant, without being accused of being Mary Sues. Shouldn't female characters have the same luxury?
I love Kelley Armstrong's Women of the Otherworld books. In the very first volume, Bitten, we meet Elena, the world's only female werewolf, and Jeremy, the current leader of the North American Pack. Both Elena and Jeremy are physically stronger than humans, with super-fast healing, severely slowed aging, and supernaturally good looks. Both of them turn into giant wolves who can eat your face. Elena, despite being the only female werewolf, is a pretty standard werewolf. Jeremy is the only non-bruiser Pack leader ever; is psychic; is rich and artistically talented and smart and his mother wasn't a werewolf at all, but a super-secret special non-werewolf supernatural and also the hottest necromancer ever loves him and and and...
Now, I think both these characters are well-written, well-rounded, and equally plausible within the setting, even if Jeremy is a bit more over-the-top than Elena is. But I've only heard the term "Mary Sue" applied to one of them. And it wasn't Jeremy. His spectacular special snowflake awesomeness is viewed as only right and fair, while her only unusual attribute—"female werewolf"—makes her, not the protagonist, but the obnoxious self-insert parasite who won't go away.
There's a problem here.
Playing Like A Girl.
Nobody has to like a girl, fictional or otherwise. But words like "annoying" or "Mary Sue" are both used as shorthand for "girl I want to dismiss." We've all read about characters who seemed overly perfect, or who had flaws the narrative wouldn't admit were flaws, and those characters are irritating. But I've seen just as many fictional boys like that as fictional girls (with the caveat that boys tend to get more pagetime, so they get more explored) and those boys don't get seen in the same way. As I was saying on twitter a couple days ago, I want characters to be flawed and awesome: I want them to be flawesome.
—Sarah Rees Brennan, Ladies, Don't Let Anyone Tell You You're Not Awesome.
So here's the thing.
When a female character is awesome, when she's the star, when she's the one the story is about, she runs the risk of being called a Mary Sue. I've had people call several of my characters Mary Sues, sometimes following it up with the all-condemning statement that clearly, these characters represent my ideal self. So you know? Toby is not my ideal self. Neither is George, or Velma, or Rose (or Sally, who you'll meet soon). Even the romantic comedy I wrote based entirely around a real trip I took to real England doesn't have a self-insert version of me as the main character; instead, it has a neurotic tech writer named Margary who likes far more adventurous food (and far more adventurous shoes). If any of my characters represents my "ideal self," it's probably Angela Baker in InCryptid, who is one of the only characters who never stars in her own book. Instead, she stays home, watches a lot of television, and does math. Heaven.
Mary Sue is a problem in a piece of fanfic. But if she's in her own story, if she's on her own stage, she can still be implausible, overly perfect, annoying, and unlikeable. What she isn't is an actual Mary Sue; what she isn't doing is warping the story to suit herself. She is the story, and that changes everything.
If you think a character in a work of original fiction is overly-perfect, say so. If you think they're overly-lucky, or overly-loved, or overly-cutesy, say so. But don't call that character a Mary Sue, or a Gary Stu, unless he or she is coming into someone else's story and warping it all out of shape (and even then, look at the context; Elphaba would be a Mary Sue in a piece of Wizard of Oz fiction, but wow is she a protagonist given her own stage in Wicked). Saying "This character is just a Mary Sue" is a way of dismissing them that isn't fair to reader, writer, or character. We can do better. We can write better. We just need to know how.
And give Mary Sue a break. I think the girl's earned it.