While there is no sure-fire, catchall way to identify a Mary Sue character type, there are some warning signs shared by the majority. One of the most universal of these signs is egregious lack of depth in a story’s primary protagonist. An interesting, enjoyable character has much going on below the surface than a story’s text reveals in the first few scenes of introduction. Personal beliefs, fears, doubts, hidden pools of strength and determination, and other such things should be discovered by the reader and the characters over the course of a story, and there should be a balance of “good” and “bad” traits that fit the character and their role in the story. A character also needs a life prior to the events of the story, unless the story is about their creation. By the end, a reader should be able to naturally think of a story’s main and side characters as real people they can sympathize with (or hate for reasons that don’t interfere with enjoying the story, in the case of villains).
Little Miss Mary Sue lacks that sort of depth, which is why she’s such a pain to read about. The writer of a Mary Sue gives little to no thought to Mary’s motivations and personal philosophies, and often reveals everything Mary Sue does possess within the first few paragraphs. The traits a Mary Sue does possess tend to be superficial - heavy emphasis on her physical features and preferred outfits is common - and laid out like a resume of “skills relevant to the job.” The list of attributes often becomes bloated with exceptionally florid verbiage to the point that it becomes obvious to the astute reader that Mary Sue and her author are trying to blind the reader to Mary’s lack of depth with a severe overdose of “look how awesome and/or sexy and/or tragically desirable I am!”
What is worse than the resume of “too much information” is the fact that, aside from a few traits that inform Mary Sue’s goals in the story and what other characters focus on when speaking about her, most of what is laid out in the introduction is rarely if ever used in the story proper. Sometimes the details can even change when a situation requiring them comes up and the author either forgot what they said about Mary Sue or realizes that the details suddenly get in the way of Mary’s perfection. So-called “flaws” are overcome in a flash of self-determination or turn out to have been advantageous all along.
If Mary Sue happens to have a backstory mentioned in their list of accomplishments, it’s usually just a collection of sympathy-garnering buzz words with little relevance to or notable influence on Mary’s present character. Mary Sue’s alleged history often includes a lack of parents or guardians, physical or emotional abuse, loss of family, friends, or home through some disaster or another, exile or ostracization due to her “specialness,” and surviving rape. Mary Sue in the present, however, rarely if ever demonstrates the kind of psychological scars that linger from such examples of childhood trauma, both because legitimate PTSD and involuntary fear reactions to triggers puts a person in states of weakness and helplessness that are incompatible with Mary Sue’s perfection and because writing believably about PTSD and similar issues is difficult even for skilled authors.
Worse still, Mary Sue’s shallowness almost inevitably infects any “established” characters the Sue’s author is writing their fanfiction about. This is reflective of an author’s lack of skill in writing deep, interesting characters, but it comes across as particularly aggravating for readers, and fans of the original work in particular, because the shallowness inflicted on the “canon” not only deprives those characters of the charm they once held but usually replaces it with excessive interest - good or bad - in Mary Sue. The other characters do not notice or shrug off Mary’s obnoxious lack of depth and may, if the author wills, attack any character who does raise a criticism about her.