Huck, a symbol? We think so. Sure, he's a great, well-rounded character—but he could also be seen as a symbol for America. (See Huck's "Character Analysis" for a few thoughts.) Check out the very last line for some pretty convincing proof: "But I reckon I got to light out for the territory ahead of the rest, because Aunt Sally she's going to adopt me and sivilize me, and I can't stand it. I been there before" (43).
In the nineteenth century, there was a lot of mythology built up around the idea of the rugged individual, the frontiersman or pioneer who was completely independent and self-sufficient, and wasn't about the let the guv'mint tell him what to do. (For "guv'mint," read "Aunt Sally" or "the Widow Douglas.") In other words, someone a lot like Huck: smart but uneducated; a little wild but fundamentally honest and moral; and not too fond of table manners.
When Huck says he's got to "light out for the territory ahead of the rest," he's taking on the role of the pioneer: heading out to new, untamed country. And we bet that as soon as it starts getting "sivilized," he's going to head out looking for yet another frontier.