As the title of the novel suggests, Tom Sawyer is the central character of the novel. Tom appears in almost every scene as the chief character. The one major exception occurs when Tom and Becky are lost in McDougal's Cave and the focus of the novel switches to Huck Finn's search for Injun Joe.
Central to Tom's character is his age. Twain deliberately did not specify his age. For many readers, Tom's age fluctuates from scene to scene. Most readers like to view Tom's age as approaching puberty--around eleven or twelve years old. If he were younger, he would not be so interested in Becky Thatcher. His fondness for Becky, while still marked by his youth (turning somersaults and otherwise acting foolish to get her attention, passing "love notes" back and forth in school, and so on), exhibits a caring and maturity that goes beyond only "puppy love." Consider, for example, his protective attitude toward her when he took the blame and punishment for her and how he cared for her in the cave episode.
Tom's character is a dynamic one, that is he moves from enjoyment in the most famous of boyhood games--playing "Indians and Chiefs," pretending to be Robin Hood, and so on--to actions that require a high degree of moral integrity. Consider, for example, his highly moral decision to break the boyish oath he took and to reveal Injun Joe's guilt in murdering Dr. Robinson--an act that freed an innocent man and placed Tom, himself, in jeopardy.
If we view Tom Sawyer simply as a boyhood adventure story, then we must assume that Twain viewed Tom erratically and used many episodes from his own youth at different times over a long period of time. Thus we have two Toms: one who plays boyish pranks on his Aunt Polly--"hooking" an apple or doughnut when she is not looking, teasing her, and finding ways to get around her--and one who has the maturity to save an innocent man and protect a frightened girl.
However, if we view Tom Sawyer as a tale of maturing, a bildungsroman--a novel whose principal subject is the moral, psychological, and intellectual development of a youthful main character--then we don't see two Tom's but one who, through his experiences, matures as a young man. Most readers then choose to see Tom as a dynamic character who occasionally reverts to childish pranks, but one who essentially moves from early childish endeavors and, when called upon to do so, matures to the point where he can make highly moral decisions and commitments, as he did in revealing Injun Joe's guilt and in protecting Becky while lost in the cave.