Copied from http://www.urbandictionary.com/:
See Mary-Sue. A female fanfiction character who is so perfect as to be annoying. The male equivalent is the Marty-Stu. Often abbreviated to "Sue". A Mary Sue character is usually written by a beginning author. Often, the Mary Sue is a self-insert with a few "improvements" (ex. better body, more popular, etc). The Mary Sue character is almost always beautiful, smart, etc... In short, she is the "perfect" girl. The Mary Sue usually falls in love with the author's favorite character(s) and winds up upstaging all of the other characters in the book/series/universe. There are several main types of Mary Sue:
Warrior!Sues: The Warrior!Sue is usually loud, obnoxious and (of course) an amazing warrior. She'll usually have some tragic past that led her to become a warrior, and she'll upstage all of the Canonical characters with her mad Sueish powerz.
Another thing to note is that a Sue will usually have a completely off-the-wall name, like "Viquetoria". The more weird and pretentious the name of the character, the more likely it is that she's a Sue.
Finally, Sues often have weird, improbable or impossible bloodlines. A secret half-elf child of Elrond and a nameless human would be an example of this. A character who was Dumbledore's grandchild and Tom Riddle's daughter would be another example.
The character in the HP fanfic was a complete and utter Sue. She moved across the street from Harry. She was beautiful, smart and pretty. They fell in wuv almost immediately. She threw all of the characters out of character and generally upstaged everyone. It was quite obnoxious.
Ok, so this isn't fanfiction; but you know what? I'm almost willing to bet it started out as fanfiction. Probably a Fringe/X-Files crossover event. The plot is liberally pilfered from a hodgepodge of older science fiction television shows. It boggles the mind that so many people think there is anything original about the plot of this book. That aside wouldn't be that big of a deal but the main character is one of the most odious and hateable characters I've come across. I liked Bella from Twilight better than this hooch, and that's saying something.