The exact opposite of a Mary-Sue or a Marty-Stu. This is a character in a book or fanfiction that is stricken by unfathomable misfortune and horrible characteristics, so much so that the entire story becomes depressing and/or unrealistic.
Person 1:"Did you read that story I wrote?"
Person 2:"Yeah...but it was really depressing. The main character seemed like kindof an Anti-Sue. Nobody has THAT much bad luck."
Person 1:"I do..."
Person 2:"No, you really don't. Stop being emo."