A fellow writing-workshop member introduced me to the Mary-Sue test.
A true Mary Sue is a surrogate for the author. But not just any surrogate… oh, no, far from it. MS is not merely a stand-in for the author. Instead, she is the embodiment of all that is true, good, and holy. She immediately wins the respect and affection of all the canonical characters, and, if the story is a romance, the undying love of whoever the writer has a crush on. She is brilliant. She is beautiful. Her hair is never out of place, even when she has a flowing mass of (fill in the blank) locks. Her career, interests, and personal beliefs are eerily similar to the author’s own. She always holds the key to the mystery. She knows how to work the computer. The slavering, vicious guard dogs curl up at her feet and gaze up affectionately. If she dies, she does so bravely and for the sake of others. In various science fiction fandoms, she occasionally saves the universe while she’s at it.
Eshva, Whatever Happened to Mary Sue, based on a definition by The Divine Adoratrice [whose link was broken when I tried to look up the original]
There’s a good entry on Mary-Sue’s in Wikipedia, and they even point you to some of the tests you can take to see whether your character is a Mary-Sue or not. My favourite is the Ponyland Express test.
One of the things about a Mary Sue is that no-one likes them.
I find it fascinating that a character the author loves so much is so intensely disliked by everyone else, particularly when these characters are based on the author him/herself.
Mary-Sues originated in fan fiction. Most of the reason no-one likes them is because they’re too perfect, and they take over the story to the detriment of other characters. Not a good thing when a fan goes in to read fiction about their favourite characters and this perfect (in every way) stranger takes over the story.
I took the Mary Sue test. My character was not a Mary-Sue, but I was warned that I had to care a little more for my character.