In fanfiction, it's very common for favoritism to show up. After all, everybody has their preferences and regardless of their role in the story, it's never a uniform thing. However, one frequent thing is for the author of a particular fanfic to appropriate a character. Rather than creating an original character, this author will overhaul the role, importance and personality of a particular canon character they like in order to make the character "ideal" (i.e., a stereotypical Mary Sue).