Are you the Main Character, the Protagonist, the Hero, or all three? And who is the Antagonist?
Depending on the story, this changes:
-Harry Potter is clearly the Main Character and Protag, but he is not always the Hero. In different books, Dumbledore and others play that role.
-Abercrombie's The First Law series lacks a central antagonist, rather it appears each Main Character is their own Protagonist and Antagonist. (Sounds tricky, but that's what I got from the books.)
-Harry Dresden is Protag, Hero and Main Character, all wrapped in one—perhaps an aspect of First Person point of view? But Bella in Twilight was the Main Character and Protagonist, but she was only the Hero for one fleeting moment. (Perhaps my issue with her.)
-I don't have an example of a Protag separate from MC and Hero, but I believe it can be done.
So how to define?
Here is my personal definition. Feel free to debate.
-The Protagonist is who is affected most by the storyline
-The Main Character is who you view the storyline through
-The Antagonist is who impedes the Protagonist the most.
-The Hero is the character in most active contention with the Antagonist.