The theme of isolation is a very important one in The Adventures of Huckleberry Finn. The title character is himself isolated by virtue of his status an an orphan, not to mention his stubborn unwillingness to conform to society's norms, customs, and values, many of which he finds utterly ridiculous and incomprehensible.
As a solitary figure, Huck has to wage an almost constant battle against the pretensions and double standards of so-called respectable society. Even when he's staying with the Widow Douglas or with the Grangerford family, surrounded by the trappings of so-called civilization, he feels completely alone. As a child of nature, Huck is destined to be estranged from society. The likes of Tom Sawyer can get closer to nature themselves as part of their games, but ultimately they will always return to the bosom of civilization. Huck doesn't have that luxury.
Nor, for that matter, does Jim. As a runaway slave, he is doubly isolated on account of his race and his status as a criminal. He too feels so much more comfortable out there in the natural world. The difference, however, is that he always needs to look over his shoulder, something that Huck, as a white boy, doesn't have to do. Nevertheless, despite their racial differences, Huck and Jim are able to forge a close bond based largely on their common estrangement from so-called respectable society and its values.