Tom's main characteristic is undoubtedly his mischievousness. He's always getting into trouble of one kind or another, whether it's playing hooky or trying to help runaway slaves escape from captivity.
Tom's not a bad kid, and he doesn't mean anyone any harm. It's just that he's blessed—or cursed, depending on how you look at it—with a deeply romantic streak that gives him an insatiable hunger for adventure. Just about everyone else sees Tom as an impudent young rascal; he, on the other hand, sees himself as a pirate or brave warrior king. Tom's nothing if not imaginative, and his already vivid imagination is fired up by the adventure stories of old.
Like most boys of his age, Tom is very superstitious and believes in all kinds of weird things that may make little or no sense to us, but which to him are deadly serious. For instance, Tom believes that a bizarre ritual involving a dead cat can cure warts. It can't, of course, but what it can do is lead Tom—and Huck this time—into even more trouble, as out in the graveyard one night they witness Injun Joe carry out a wicked murder.
Despite his penchant for superstition, Tom's actually quite a smart kid. Not book smart, maybe—he really hates going to school—but certainly street smart. Tom's intelligence is more practical than intellectual. His mind is always going in several different directions at once, forever cooking up all kinds of plans; some good, some bad.
An example of the former would be when he shows remarkable ingenuity in saving himself and Becky Thatcher from almost certain death in the caves where they get lost. An example of a bad plan would be when Tom gets some of the neighborhood kids to whitewash the fence for him, which was his punishment from Aunt Polly for skipping school to go swimming.