A Mary Sue is a character in a work of fiction who exists primarily for the purpose of wish-fulfillment on the part of the author. She plays a prominent role in the work, but she is notably devoid of flaws or a complex personality, and she usually represents the pinnacle of idealized perfection. All of the other characters love Mary Sue, because she is extraordinarily helpful, talented, beautiful, or unusual, and she often drives readers absolutely crazy because she is one-dimensional and too idealized to be realistic. The male equivalent of a Mary Sue is a Gary Stu.
While Mary Sues have been around in fiction for quite a long time, the modern term is derived from a parody of Star Trek fan fiction written in the 1970s. A certain Lieutenant Mary Sue was a thinly veiled idealized version of the author, and the name came to be used for any such character. Novice authors in particular are at risk of creating Mary Sues, and several websites publish litmus tests which can be used to determine whether or not a character is a Mary Sue.
For readers, the Mary Sue can be a uniquely frustrating character. Her lack of depth and idealized perfection make her rather uninteresting, and sometimes so implausible that she ruins the story. Mary Sues often break the rules of a fiction world, and the flowery descriptions of their perfect beauty and profuse talents can grow dull for the reader. Many Mary Sues are also extremely cliched, and authors use cliched terms to describe them; they have “glowing eyes,” “flowing hair,” “porcelain skin,” and so forth. They have unique magical powers, the capability to defy the laws of physics, and an improbable assortment of talents; a Mary Sue can be a doctor, a ballerina, and an astronaut all in one.
While representations of the author are present in many works of fiction, a Mary Sue is not a real representation of the author. Many authors explore facets of themselves in their work, probing into their flaws, weaknesses, and history to strengthen a piece of fiction and make it more engaging. Readers find these characters interesting because they are realistic, flawed, and accessible, making the reader feel a deeper connection with the story. A Mary Sue, on the other hand, is such an obvious insert that she often ends up alienating the reader.
The genre of fan fiction is notorious for its Mary Sues, and the sentiment is certainly understandable; of course fans would want to imagine themselves in the fictional worlds they enjoy so much. However, Mary Sues also appear in mainstream fiction, film, and television, and astute reviewers often point this out when criticizing such work.