Mary Sue is a character trope more commonly found in fanfiction, although they can also be found in original stories. Here are some comments made about Mary Sue:
A female character who is so perfect that she is annoying. The name originated in a very short Star Trek story that mocked the sort of female characters who showed up in fanfiction. It usually refers to original female chaacters put into fanfiction but can refer to any character. Mary Sues are characters who are usually extraordinarily gorgeous, amazingly talented, unusually powerful, and exceedingly attractive to whoever the author has a crush on. They often possess ridiculously fancy and pretentious first names -- Angel, Raven, Jewel, Lorelei Bianca Julia Marizza Snape -- and are very, very annoying.
TV Tropes has a page for her:
Mary Sue on TV Tropes
A Mary Sue (sometimes just Sue), in literary criticism and particularly in fan fiction, is a fictional character with overly idealized and hackneyed mannerisms, lacking noteworthy flaws, and primarily functioning as a wish-fulfillment fantasy for the author or reader. It is generally accepted as a character whose positive aspects overwhelm their other traits until they become one-dimensional. While the label "Mary Sue" itself originates from a parody of this type of character, most characters labelled "Mary Sues" by readers are not intended by authors as such. Male Mary Sues are often dubbed "Gary Stu", "Larry Stu", "Marty Stu", or similar names.
While the term is generally limited to fan-created characters, and its most common usage today occurs within the fan fiction community or in reference to fan fiction, original characters in role-playing games or literary canon are also sometimes criticized as being "Mary Sues" or "canon Sues" if they dominate the spotlight or are too unrealistic or unlikely in other ways. One example of this criticism is Wesley Crusher from Star Trek: The Next Generation.
The thing is that everyone bashes the Mary Sue, but how many people offer to help fix her? I haven't heard of anyone doing so. Therefore, I am going to undertake to fix a Mary Sue.
4.) Interpersonal Relationships