Protagonist
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Character Role Analysis
Frodo, Aragorn, Merry/Pippin
With three stories it only makes sense to have three protagonists. We'll start with Frodo because, well, he's Frodo. If there is a main character in LotR, he's the one. We follow our Ring bearer through hazardous terrain toward the Black Gates. Both Sam and Gollum refer to him as master. They are his guide, guard, and gardener that support him along his journey with advice and friendship and keeping that dang Ring off his finger… even when he really, really wants to put it on.
Then there are Merry and Pippin who just sort of… do their own thing. They're kind of our protagonists by default. They have their own story through which we get to see Treebeard and the ents rise up against Saruman.
Not that they're just along for the ride (although they do spend most of their time riding Treebeard). Pippin plays an important role in convincing Treebeard to involve the ents in the war when he asks to be taken toward Isengard. Hey, Pippin actually does something useful and intelligent for once. Take that, Merry!
And then we have Aragorn, our throne-less king who seems to be playing a bigger and bigger role as the story progresses. In the first film he was a leader, helping the hobbits reach Rivendell and then taking over as head of the fellowship when Gandalf fell.
But in The Two Towers we're getting even more characterization. He almost dies but returns, develops a relationship with Erwin, has flashbacks to his talks with Arwen and Elrond, and is generally given much respect by Théoden and others because of his lineage. Aragorn is becoming less and less of a nobody ranger and more of a king. Given the title of the final film, we don't expect this to change.