A work of fan fiction in which the writer of said fan fiction decides to replace the hero/heroine with him/herself. Because it is essentially copying a character's life like copying someone's diary, such fan fiction is unimaginative plagiarism. People who write protagonist plagiarism fan fictions are usually uncreative and incapable of writing their own stories. A person who is a protagonist plagiarist is also a sub type of Mary Sue that only writes for his or her own self gratification.
That loser is so desperate that she wrote a fan fiction that was protagonist plagiarism!