A protagonist: a person who knows s/he wants something badly enough to fight for it. As working definitions go, that’s not bad; and it’s all about the fight, the effort, the commitment.
If you want to fight for something, you either believe you deserve it or that it’s morally right. If you do fight for something, it's foremost in your intentions; you exert planning, effort, and you will have a defining experience of yourself in real terms. (Reality’s the pay off for every interesting protagonist.)