Huck Finn is, on some level, everything Tom wants to be. Huck doesn't have to go to school or wear shoes or do any of the other things adults make kids do. He's also always game for whatever crazy scheme Tom has dreamed up. Want to go live on and island and be pirates, Huck? Sure thing. Want to go treasure hunting? Of course. Want to put your life on the line and follow around two hardened criminals. Definitely.
Huck usually defers to Tom, especially when it comes to the particulars of being a pirate or a robber. He trusts Tom and, as in the case of Muff Potter, is willing to make – and keep – a promise with him. He even learns a thing or two from his buddy. If Tom hadn't stood up and testified on behalf of Muff Potter, Huck may have never gotten up the courage to tell the Welshman about Injun Joe.
Huck's there with Tom when they're nearly caught by Injun Joe, and he's got his back when they finally get the treasure. Though he's often reluctant to embark on Tom's adventures, he doesn't chicken out when the going gets tough.
Honorable Mention goes to Joe Harper, Tom's "bosom friend," military rival, and co-adventurer. Unfortunately, Joe just sort of drops off the radar in the second half of the book, through no fault of his own.