Where the term comes from
Wikipedia traces the name to the main character of "A Trekkie's Tale", a short parody by Paula Smith published in the Star Trek fanzine Menagerie. The story mocked the idealised young heroines common in Star Trek fan fiction, who were often adored by the established characters and central to the plot despite not appearing in the source material. Joan Verba listed typical features: being a young or the youngest officer, being loved by the established cast, having extraordinary abilities, winning extraordinary honours and dying a heroic death that everyone mourns. The label has since spread beyond fan fiction and become mostly pejorative, and a similar male character is sometimes called a Gary Stu or Marty Stu.
TV Tropes adds that there is no consensus on a precise definition, that the term has been applied differently in different circles, and that it often points to an author avatar or an over-skilled, over-idealised character. That vagueness is useful to know before you use the label on someone else's work or fear it in your own.
An example
Wrenna Pike, in an early draft, is a sixteen-year-old who can duel, heal and decode the old script. Every elder trusts her at once, and the rival she insults ends up falling for her. Nothing she attempts costs her anything. In the revision, her skill with a blade comes from a childhood spent as a fencer's errand girl, and it is the only thing she can do. She misreads the old script and sends a patrol to the wrong valley. The elders distrust her until she pays for the mistake. She is still capable, but now readers can watch her earn the trust that used to be handed to her.
Common mistakes
- Giving the character every talent so that the plot never tests them.
- Making every other character adore the lead without reasons.
- Adding "flaws" that are really strengths, such as caring too much.
- Calling any capable or admired protagonist a Mary Sue. Competence is not the problem; the lack of cost is.
- Writing the character as a wish-fulfilment stand-in and then hiding that from the reader, who will notice.
How to check your manuscript
Write down three things your protagonist is bad at, and find a scene where each one causes harm. Then list who admires or fears them and why, and check that each of those reactions is earned on the page. Ask whether the plot would change if the character were replaced by a different capable person. For related terms, see protagonist, stakes and character arc.
In Musael, world and characters lets you link entries by relationship, so you can see at a glance whether anyone in the book disagrees with your lead.