Real life can bring disappointment. Santa Claus is a Catholic bishop who punched heretics. And Mary Sue, that figure beloved by amateur writing lore, isn’t real. She and her stepbrother Gary Stu are just figments of the amateur writing wars.
Mary Sue, other than being a way of making fun of women with unfashionable names like Mary and Susan, has no objective meaning. One person says it’s a character that is too perfect. (Was Jesus Christ then the ultimate Mary Sue— or Gary Lou?) Another says it’s a character who is too ‘nice.’ Or a character that doesn’t have the right flaws— the ones that ‘count.’
Realistically, calling someone else’s character a Mary Sue is another arena to fight the opinion wars. John’s a hardcore angry atheist? Then every character in your Evangelical Christian romance is a Mary Sue because they all go to church weekly and refrain from stealing and using heroin. Does Mandy have a low opinion of the ‘politically correct?’ Then she will accuse your characters of being sensitive-snowflake Mary Sues who will worry if a murderer would think it’s ‘racist’ if he is asked not to do the murder thing any more. Any character more morally straight than Jeff Lindsay’s Dexter Morgan can be seen as a Mary Sue by somebody.
If you get accuse of having written a Mary Sue, perhaps the best thing to do is ignore it, as you would do if someone accused your protagonist of being Santa Claus’s lead reindeer. If you really feel you have to respond, ask the person— what, specifically, is wrong with the character? If he responds ’she’s too nice,’ that’s not a specific response. In what scene, in what action, is that character being ‘too nice?’ Chances are, it’s going to come down to a matter of taste or opinion.
Sometimes the problem is genre. If you are writing Christian fiction or Amish romance, the guy who writes spy novels with high body counts may see all your best characters as Mary Sues. But the genre standards are different. If you write an Amish girl who can kill bad guys seven different ways with a pencil eraser, that won’t meet the standards of the Amish romance genre, which has a notoriously low body count.
When you despair, remember that the greatest writers in the world wrote characters who were nice as well as ones that were Lady Macbeth. Jane Austen wrote whole books full of people who never called anyone a motherf-ck-er, not once. Was she a writer of Mary Sue characters? No. And neither are you. Go forth, and write stuff!