If you're wondering what the heck I'm talking about, don't worry, I'd never heard of it either, but apparently, in literary criticism, a'Mary-Sue'is a fictional character the writer has created who...you know what, I really can't be bothered to explain so here's what Wikipedia has to say about it:
A Mary Sue (or Gary Stu), is a character with overly idealized and hackneyed mannerisms, lacking noteworthy flaws, and primarily functioning as wish-fulfillment fantasies for their authors or readers. Perhaps the single underlying feature of all characters described as 'Mary Sues' is that they are too ostentatious for the audience's taste, or that the author seems to favor the character too highly. The author may seem to push how exceptional and wonderful the 'Mary Sue' character is on his or her audience, sometimes leading the audience to dislike or even resent the character fairly quickly; such a character could be described as an 'author's pet'.
Someone has even devised what is known as The Mary-Sue Litmus Test. Be warned, it's awfully long.
Did I learn anything from it? Well, not really. It's a basic principle when creating characters. Make them likeable, well-drawn and not cliched...end of. Simples!