See Mary-Sue. A female fanfiction character who is so perfect as to be annoying. The male equivlalent is the Marty-Stu. Often abbreviated to "Sue". A Mary Sue character is usually written by a beginning author. Often, the Mary Sue is a self-insert with a few "improvements" (ex. better body, more popular, etc). The Mary Sue character is almost always beautiful, smart, etc... In short, she is the "perfect" girl. The Mary Sue usually falls in love with the author's favorite character(s) and winds up upstaging all of the other characters in the book/series/universe. There are several main types of Mary Sue:

Victim!Sues: The Victim!Sue is your whiny, wimpy, pathetic female character who can't seem to do much of anything except cry and get herself into trouble that the romantic interest of the fic has to rescue her from.

Warrior!Sues: The Warrior!Sue is usually loud, obnoxious and (of course) an amazing warrior. She'll usually have some tragic past that led her to become a warrior, and she'll upstage all of the Canonical characters with her mad Sueish powerz.

Mage!Sue: Similar to the Warrior!Sue, the Mage!Sue has amazing stregnth in magic, or has a magical power that nobody else has. She'll usually wind up upstaging all of the magical charac...

The most common type of Mary Sue is a character based an the author's idealization of themself. Furthermore, because the author is imagining a preferred version of themself, and because faults are overlooked in favor of optimization, a Mary Sue tends to have only superficial resemblance to the author, sharing similar likes/dislikes and a similar spirituality (when applicable), but objectifying things such as worldview and relationships. This type of Mary Sue is more common simply because it's easier to write and is more appealing to the author. This Mary Sue is found in fanfics and original fiction alike. Yes, Mary Sues abound even in professional writing.

The other type of Mary Sue is a character intended to be an ideal match for another, appearing almost exclusively in fanfics. In this case, "ideal match" means that the character's positive traits are exaggerated to render impossible any competition for the love interest. Arguably more pernicious than the "self-idealization" type, the "ideal match" type by its very nature prevents compelling character or plot development, which the "self-ideal" Mary Sue may be able to avoid.

A Mary Sue or Gary Stu (male form) is often either a perfect or almost perfect character. They often appear in fan fictions and are sometimes based on the author themselves.

If the Mary Sue or Gary Stu is based on the author, they normally have qualities that the author wishes they had.
For example, they wish to be more popular or better looking.

When a Mary Sue or Gary Stu is written into a fan fiction, it will often cause canon (original) characters to become OOC (out of character). They often have terrible, grief-filled pasts and 'need' a certain canon character to understand them or they have a past that is the exact same or very similar to a canon character's.

Mary Sues and Gary Stus are mostly found in new writers or written in fan fictions by more exprerienced writers for fun.

They also tend to have either very common or extremely strange and exotic names or, if they are made for fun, will sometimes have the name Mary Sue or Gary Stu.