Mary Sue: noun.
A female character who is so perfect as to be annoying. A Mary Sue character is usually written by a beginning author, or one who is too lazy to make an actual effort. Often, the Mary Sue is a self-insert with a few "improvements" (ex. better body, more popular, etc). In short, she is the "perfect" girl. i.e., a perfect, "desu" female character, often with huge boobs, neko attributes, demon blood, etc.