I recently found myself down the fanfiction rabbit hole—that is, somehow, I ended up reading things I didn't even look for. But that's ok, because I was reading some fascinating essays on how not to write Mary Sues, specifically in the Harry Potter universe. Some were nice guides or lists, some were essays, and there was the occasional AU story in which canon characters explained what Mary Sues do to their universe. I found most of these helpful and interesting—and some of them were really smart!—and decided I would write on the problem, as well. Despite the fact that I found countless how-tos on the subject, I figure the more written, the more accessible they are, which means more writers might read them and heed the advice.
Personally, I don't really mind Mary Sues if the story is handled decently. I've read handfuls of fanfics in which I would deem the original character (OC) a Mary Sue, and all of the other characters out of character (OOC) yet I still enjoyed the story. Good story telling is not all character, but plot and good writing. I write Mary Sues—Ivy Blythe from my story A Shine to It is easily classified as a Mary Sue. But I digress. I will address the merits and detriments of Mary Sues at the end. Most of this is about how to avoid writing a Mary Sue.
So, let's get on with talking about Mary Sues, or their male counterpart, Gary Stus. For the sake of simplicity, however, I will continue writing using only the term Mary Sue, as the female rather than male, because 1. I see a lot more Mary Sues than Gary Stus, 2. It's a lot of extra words. 3. Most of what I will address can be applied to the other gender, so if you're looking to avoid a Gary Stu, this can be applicable to you.
WHAT IS A MARY SUE
Mary Sues have several similar definitions, but the most simple, comprehensive one I found was on the TVTropes website (a website I highly recommend for writers and readers to explore in order to understand fanfiction cliché). Here is what they wrote:
"The prototypical Mary Sue is an original female character in a fanfic who obviously serves as an idealized version of the author mainly for the purpose of Wish Fulfillment. She's exotically beautiful, often having an unusual hair or eye color, and has a similarly cool and exotic name. She's exceptionally talented in an implausibly wide variety of areas, and may possess skills that are rare or nonexistent in the canon setting. She also lacks any realistic, or at least story-relevant, character flaws — either that or her "flaws" are obviously meant to be endearing.
She has an unusual and dramatic Back Story. The canon protagonists are all overwhelmed with admiration for her beauty, wit, courage and other virtues, and are quick to adopt her as one of their True Companions, even characters who are usually antisocial and untrusting; if any character doesn't love her, that character gets an extremely unsympathetic portrayal. She has some sort of especially close relationship to the author's favorite canon character — their love interest, illegitimate child, never-before-mentioned sister, etc. Other than that, the canon characters are quickly reduced to awestruck cheerleaders, watching from the sidelines as Mary Sue outstrips them in their areas of expertise and solves problems that have stymied them for the entire series. (See Common Mary Sue Traits for more detail on any of these clichés.)
In other words, the term "Mary Sue" is generally slapped on a character who is important in the story, possesses unusual physical traits, and has an irrelevantly over-skilled or over-idealized nature."
So, if TVTropes explained it so well, why am I giving it a shot? Well, I'm focusing solely on Harry Potter Universe fanfiction. I will be going through common traits of a Mary Sue, offering examples and ways to avoid Mary Sues, and end with an argument for Mary Sues and some tips for good writing in this particular type of fanfiction.