Frodo Baggins
Frodo carries the Ring toward Mordor with Sam and Gollum as his only companions. He grows more burdened as the book goes on. His decision to trust Gollum drives the second half of the plot, and his capture by orcs ends the book in crisis.
Samwise Gamgee
Sam distrusts Gollum from the start and is right to. He fights Shelob, takes the Ring when he thinks Frodo is dead, and gives it back the moment he can. His loyalty is the most consistent force in the Frodo storyline.
Gollum
Gollum guides Frodo and Sam because he wants the Ring back. He is genuinely useful as a guide and genuinely dangerous as a companion. By the end of the book, he has reverted to his murderous Sméagol self and led them into Shelob's lair.
Aragorn
Aragorn leads the pursuit of Merry and Pippin, fights at Helm's Deep, and confronts Saruman at Orthanc. He is the military and moral center of the Rohan storyline, though he is not yet king.
Gandalf the White
Gandalf returns from death with greater authority and power. He breaks Saruman's hold on Théoden, commands the battle at Helm's Deep, and strips Saruman of his rank. His return shifts the balance of the war.
Merry and Pippin
Captured by orcs, they escape into Fangorn and befriend Treebeard. Their relationship with the Ents leads directly to the destruction of Isengard. They are no longer passengers in the story.
Treebeard
The oldest Ent, slow to decide but devastating once committed. He leads the march on Isengard after Merry and Pippin convince him that Saruman's destruction of the forest cannot be ignored.
Théoden
King of Rohan, mentally dominated by Gríma Wormtongue and, through him, Saruman. Gandalf breaks that hold and Théoden reclaims his role as king. He leads his people at Helm's Deep and begins the arc that continues into the third book.
Faramir
Captain of Gondor's rangers and Boromir's younger brother. He captures Frodo and Sam, learns about the Ring, and refuses to take it. His restraint is one of the clearest moral contrasts in the book.
Gríma Wormtongue
Saruman's agent inside Rohan's court. He has been feeding Théoden despair and bad counsel. After Gandalf expels him, he flees to Orthanc and later throws the palantír from the tower, which sets up Pippin's dangerous encounter with Sauron.