Tom Sawyer is a young man living in St. Petersburg, Missouri, with his aunt Polly and half-brother Sid. His regular activities include testing the rules and seeking adventure with his friends. Tom is clever, easily able to manipulate his peers and his aunt. His brother and his teacher, Mr. Dobbins, both see through his ruses. His aunt does, too, but she wants to believe the best of him. Becky Thatcher, his love interest, also wants to believe him. He is at heart a good boy and ultimately chooses to do the right thing on major issues (confessing that he is alive and revealing the murderer), but in day-to-day matters Tom is less strict with his adherence to right and wrong.
Huckleberry Finn (Huck) is in many ways a less "civilized" version of Tom. His father is a drunkard, and so Huck has been left to his own devices and is essentially raising himself. He's kind to enslaved people and willing to risk himself to do the right thing. He's also eager to avoid the trouble that comes from doing so. He asks the Welchman not to reveal his involvement in protecting the Widow Douglas, and he does not come forward to testify for Muff Potter. Huck does not seek a proper home or nice clothes (casting them off when he's made to wear them at the novel's conclusion). His pursuit of treasure with Tom seems no different from any of the other games they play. In the end, he'd rather throw away the money than accept the social rules and limits that it brings to his life.
The two traits that most often characterize Aunt Polly are her kind heart and her gullibility. She cites religion in disciplining Tom, and even as she criticizes him she indulges him. She is eager to see the best in him.
Becky alternates between rejecting Tom's advances and wanting his attention. She is in many ways the right match for him. She sees through his manipulations but—like Aunt Polly—continues to be fond of him. Moreover, Becky is willing to break the rules. She steals an anatomy book from Mr. Dobbins, and she's willing to go on an adventure in the cave with Tom. That said, Becky is also quick to cry and allow Tom to solve her problems. This is true both with the punishment for the book's theft and when they are stranded in the cave.
Joe Harper is one of Tom's closest friends. He is a regular companion on Tom's adventures and happily engages in make-believe games of pirates or Robin Hood. However, Joe was not a witness to the murder in the graveyard, so he is at times seemingly less involved in Tom's life. He does run away with Tom and Huck. Joe has an intact family, as opposed to Huck and Tom. This may account for the apparent distance between the characters. Joe has more rules he must follow because of his more conventional household and upbringing.
Injun Joe is a stereotypical representation of a Native American. He's a man bent on revenge in more than one situation. He strikes down the doctor, murdering him for a slight in the past. His intent to attack the widow is also based on his anger about a past transgression (in this case committed by her now-deceased husband). Injun Joe lies in order to have Muff Potter take the blame for the murder he commits. He steals. He is, to the boys, the worst of threats. The novel does not give us reason to doubt their assessment. Injun Joe has a cache of treasure and weapons, a result of past crimes. It is this treasure that Tom and Huck eventually claim as their own. Despite all of his crimes, Injun Joe's death is an accident. He is locked in the cave and starves to death while Tom is recovering from his ordeal. Joe's death is necessary for the boys' safety, but they are not actively responsible for it.