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 cliches.) 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.