Tom’s well-behaved yet vindictive brother Sid highlights all of the ways that Tom rebels against authority, making him a foil. For example, when Tom plays hooky from school, Sid tells on him for doing so. When Tom neglects to say his prayers one night, Sid “made mental note of the omission.” Sid’s presence inspires Aunt Polly to compare the boys, which leads her to find Tom wanting, such as during the scene when she raps Tom on the knuckles for sneaking sugar:
He said: “Aunt, you don’t whack Sid when he takes it.”
“Well, Sid don’t torment a body the way you do. You’d be always into that sugar if I warn’t watching you.”
Presently she stepped into the kitchen, and Sid, happy in his immunity, reached for the sugarbowl—a sort of glorying over Tom which was well-nigh unbearable.
Throughout the novel, Huck Finn is a foil for Tom, meaning that his presence is used to reveal information about Tom. While Tom acts rebellious but returns to a safe home and a loving aunt at the end of the day, Huck is truly an outcast—he has no home, no parents (his mother is dead and his father is an alcoholic who often disappears), and he doesn’t even attend school. Twain describes how Huck’s status as a social pariah is precisely what makes parents afraid of him and boys like Tom want to spend time with him:
Unlock with LitCharts A+Huckleberry was cordially hated and dreaded by all the mothers of the town, because he was idle and lawless and vulgar and bad—and because all their children admired him so, and delighted in his forbidden society, and wished they dared to be like him. Tom was like the rest of the respectable boys, in that he envied Huckleberry his gaudy outcast condition, and was under strict orders not to play with him. So he played with him every time he got a chance.
Injun Joe reveals that all of the romantic adventures Tom dreams about having (robbery, treasure-hunting, living life outside of authority, etc.) are not as lighthearted as they look, making him a foil for Tom. While Tom imagines himself to be an outcast in trouble with the law, Injun Joe actually is, and his life is not enviable—there are real consequences for the ways in which he impulsively murders people and hunts for treasure (namely, that he is hunted down and eventually dies).
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