A Mary Sue/Gary or Marty Stu story is one that's a self-insert, usually done subconsciously and with no knowledge on the writer's part (until they get a bad review, that is). Most Mary Sues are badly written, but some have been written well enough that they eventually developed into rell-rounded characters. It is possible to make a canon character into a Mary Sue in a fanfic.
Some characters have been Mary Sues in canon. One example is Wesley Crusher in Star Trek: TNG. Wesley was Gene Roddenberry's middle name.
I believe there's a hero in all of us that keeps us honest, gives us strength, makes us noble, and finally allows us to die with pride, even though sometimes we have to be steady and give up the thing we want the most. Even our dreams. -- Aunt May, Spider-Man 2
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