Mary Sue

A Mary Sue is a perjorative form of a fictional character in fanfiction (mostly perhaps Fan Fiction) which is overly "perfect". A Mary Sue will be lacking noteworthy flaws, or may have flaws which will be intended to make the character more likeable, and will be too unrealistic. No living human is perfect in every way, so neither should a fictional character be.

Male Mary Sues are often called Gary Stus or Marty Stus. Mary Sue is the general term since female Sues are more common.

Warning Signs
Mary Sues will often, but not always:

In FanFiction

 * Will be related somehow (long-lost sister or something) to character(s) from the canon.
 * Will be instantly able to make canon characters fall in love with them.

Personality and Appearance

 * Be perfect in every way. Smart, strong, brave, beautiful/handsome, witty, etc.
 * Have a natural hair or eye color that does not occur among their people or race. A pink-eyed human would fall into this category, but if the character belongs to a race where pink eyes are common, then it's okay.
 * Get a lot more description compared to the other characters, usually having to do with their appearance. This is because of the author's unnecessary efforts to show the reader how wonderful the character is.

Flaws (or lack thereof) and Skills

 * Have very few flaws, or even none at all. If they have flaws, it is likely that either they will be trivial and unimportant to the story (like the inability to swim in a desert adventure story), the author will say that the character has a flaw, but not show it in his/her actions, or the flaw will not actually get the character into trouble in the story. If clumsiness is only used to allow other character's to grab Clumsy's arm to stop him/her from falling, or just to appear "cute" to other characters, it doesn't constitute a sufficient flaw. If said character were to accidentally break something valuable (and get in trouble for it), constantly be tripping and getting hurt, or trip and fall during a fight or a chase, then it would constitute a sufficient flaw.
 * Be skilled in everything, even things that are irrelevant to the setting.
 * Such as being able to sing or dance in a swords-and-sorcery battle story.
 * Be able to pick up a complex skill (swordfighting, gymnastics, etc.) in a short amount of time.

Past

 * Have a tragic past, often involving physial, emotional, or sexual abuse, imprisonment, or loved ones dying or being taken away.
 * Suffer guilt for something they did in the past, even though it is clear that it wasn't their fault.

Name

 * Have names which are misspellings of other common names (Jennyfer, Daryk, Mariia). K's and Y's stuck in odd places are common as well.
 * Have names which are overused, often ones that are a noun, verb, or adjective not commonly used as a name or ones that somehow tie in with the character's personality or other characteristics (Raven, Ebony, Luna). Remember, their parents will not have known how their baby's personality would develop when he or she was born, so unless this character lives in a society where the name is chosen at age 10 or something, or if they changed their name, it likely won't fit their personality.
 * Have names which are inappropriate for the setting, like a Japanese name in a pseudo-medieval society.

Role in the Story

 * Die a tragic and emotional death after sacrificing themselves.
 * Never have to make sacrifices to get what they want, excepting the above example.
 * Be a Chosen One.

Interaction With Other Characters

 * Be liked by all.
 * Have many other characters fall in love with them, or have sexual relations with multiple characters.
 * Other characters will often rhapsodize about how beautiful/handsome they are, even characters who normally wouldn't.

Author Interaction

 * A character may dismiss something that happened in Mary Sue's past, only to have said trauma happen to him/her.

Note that a character can have a couple, several, or even a lot of these traits and not be a Mary Sue. They can even score high in litmus tests (see below) but still not be a Mary Sue. These are just common traits of Mary Sues. However, it takes skill to do these things without making the character a Mary Sue.

Anti-Sue
See Main Article

Is My Character a Sue/Stu?
There are ways to find out. There are several online "litmus tests" which you can use to see if a character is a Mary Sue. These are the most popular.
 * A test for characters in original fiction.
 * A simple, shorter litmus test.