Mary Sue and Gary Stu

A Mary Sue (or Gary Stu), is a character with overly idealized and hackneyed mannerisms, lacking noteworthy flaws, and primarily functioning as wish-fulfilment 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 favour 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'.

(My apologies that the above definition is quoted verbatim from someone's blog but I didn't make a note of whose it was! If it was you please let me know and I'll change this to an acknowledgement.)