Protagonist
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Character Role Analysis
Marty McFly
Well, the amount of time MM is onscreen is a pretty big clue. But really, we know Marty's our protagonist because this is his story, his ordeal, and…we're rooting for him.
He's got a number of obstacles he needs to overcome—making his way back to the future, first and foremost, but also getting over his fear of rejection, building a better relationship with his parents (or at least forgiving them for their shortcomings), leaving behind his "slacker" status, etc. And because he's such a charismatic, likeable guy, we want him to accomplish all those things.