A character who's so idealized and flawless that they break the story's believability, often a stand-in for the author's fantasies.
A Mary Sue is a character who seems to exist without meaningful limitations. They're exceptionally talented, universally loved (or feared), and the plot bends around them like gravity around a star. The term originated in 1973 from a Star Trek fan fiction parody by Paula Smith, where 'Lieutenant Mary Sue' was a fifteen-and-a-half-year-old officer who was the youngest, smartest, and most beloved person on the Enterprise. The concept has since expanded well beyond fan fiction, though it's worth noting that the label gets applied unevenly - female characters are called Mary Sues far more often than equally overpowered male characters, which has sparked legitimate debate about whether the term carries gender bias.
Understanding the Mary Sue concept helps you spot when a character has become a wish-fulfillment vehicle rather than a believable person. The real problem isn't competence - it's the absence of meaningful struggle, real consequences, and genuine flaws that create tension. Your readers need to worry about your characters, and they can't worry about someone who always wins. That said, be careful about weaponizing this label. Sometimes a character is just competent, and that's fine.
Meg Murry is a great counter-example - she's gifted but also stubborn, insecure, and angry, making her feel real rather than idealized.
Wade Watts has drawn Mary Sue criticism for his encyclopedic knowledge conveniently matching every challenge he faces.
Edmond Dantes post-prison is arguably a Gary Stu - impossibly wealthy, cultured, and brilliant - but the story works because his perfection masks deep brokenness and moral corruption.
Apply the same standard regardless of gender. The real question is: does the character face genuine challenges, make real mistakes, and earn their victories? If yes, they're probably just a capable character.
Real flaws create real consequences. A meaningful flaw should cause the character to make bad decisions, hurt people, or fail in ways that actually affect the plot.
Characters should be good at things - that's fine and even necessary. The key is balance: let them struggle, fail, and earn their growth through genuine effort and cost.
Take your most capable character and write a scene where their greatest strength becomes their biggest problem. If they're brilliant, let their overthinking paralyze them. If they're charismatic, let their charm backfire. Write 300 words where competence creates genuine trouble rather than solving it.