Mary Sue From Wikipedia
Mary Sue, sometimes shortened simply to Sue, is a pejorative term used to describe a fictional character who plays a major role in the plot on such a scale that suspension of disbelief fails due to the character's traits, skills and abilities being tenuously or inadequately justified. Such a character is particularly characterized by overly idealized and clichéd mannerisms, lacking noteworthy flaws, and primarily functioning as wish-fulfillment fantasies for their authors. Perhaps the single underlying feature of all characters described as "Mary Sues" is that they are too ostentatious for the audience's taste, or that the author seems to favor the character too highly. The author may seem to push how exceptional and wonderful the "Mary Sue" character is on his or her audience, sometimes leading the audience to dislike or even resent the character fairly quickly; such a character could be described as an "author's pet".
"Mary Sues" can be either male or female, but male characters are often dubbed "Gary Stu" or similar names. While the label "Mary Sue" itself originates from a parody of this type of character, most characters labeled "Mary Sues" by readers are not intended by authors as such.