Eustace Scrubb
Eustace starts as the book's most difficult character on purpose. He is selfish, dismissive, and convinced he is smarter than everyone around him. His dragon transformation and restoration by Aslan make him the book's central arc. By the end, he is changed enough that Edmund notices and says so.
Edmund Pevensie
Edmund is steady and experienced here compared to the earlier books. He acts as a grounding presence on the ship and mentors Eustace after the dragon episode. His near-loss of control at Deathwater shows Lewis is not letting him off the hook entirely.
Lucy Pevensie
Lucy is warm and perceptive, but the magician's house sequence shows she is not immune to temptation. She wants to be more beautiful and she eavesdrops when she should not. Aslan corrects her without crushing her, and she moves forward.
King Caspian
Caspian is the ship's captain and the quest's leader. He is brave and mostly just, but the Deathwater scene shows he can be pulled toward greed and pride. His decision at the End of the World — to abandon the quest and sail on — has to be overruled by his crew.
Reepicheep
The talking mouse is the book's moral compass in motion. He is courteous, fearless, and completely fixed on reaching Aslan's country. He never wavers. Lewis uses him to show what it looks like to want the right thing without reservation.
Aslan
Aslan appears at key moments rather than traveling with the crew. He tears off Eustace's dragon skin, dissolves the greed at Deathwater, and appears to Lucy in the magician's house. He is the force that corrects and restores, but he does not prevent the characters from getting into trouble first.
Lord Rhoop
One of the seven lost lords, found on the Dark Island in a state of madness after his nightmares became real. His rescue is one of the most urgent scenes in the book and shows what prolonged fear does to a person.