“You already know Mary Sue. Mary Sue is the perky, bright, helpful sixteen-year-old…. [Everyone] likes Mary Sue, because Mary Sue is good at everything… Mary Sue often has mental powers that may manifest themselves as telepathy, precognition, or magic. Her past is tragic, more so than any other character… If Mary Sue is very young, she is often the offspring of one or two already established characters. If she's a little older, she will probably end up sleeping with the author's favorite character. Sometimes, she fills both roles. Her name is often the author's name, be it a net.name, a favored nickname, or the author's middle name...”
Japanese names, special powers, strange markings, oddly colored hair and/or eyes, angsty pasts, exceptional beauty and/or talent, and being a mixed species are all signs of a Mary-Sue. Normally only the author (who more often than not is a preteen and doesn't know any better) likes the Sue, the readers all find her incredibly annoying and unrealistic. Almost every skilled author has a few Mary-Sues in their closet, from a time when they were first starting out, so if you’ve ever written a Mary-Sue fic, don’t feel bad, everyone has to start somewhere. If you fear perhaps your character falls into the realm of Sue-dom, you can take The Mary-Sue Litmus Test, and the Harry Potter Sue litmus test to double check. If she does, your character may need a bit of editing.