A gorgeous, athletic, talented and intelligent girl who has every man on earth chasing after her. Even though she is beautiful, she will deny this. Almost always on the good side. If not, she's that overpowered hottie on the other side. Unusually attractive, and is related or in a relationship with any Canon character.
So perfect that it's very annoying.
Used most commonly in RPGs, Fan Fictions etc etc.
Her male counterpart is Gary-Stu
WARNING If you character is named Mary Sue but does not have these traits, the name does not exactly matter. It is okay to give you character the name, Mary Sue.