Huck and Jim are both naive and inexperienced when it comes to the "ways of the world". While Jim is much older than Huck, they have both been shielded from the outside world, Huck by his age and Jim by his circumstances.
They are both gullible and supersititious. Jim honestly believes Huck is a ghost when he first sees him on Jackson's Island. Huck, while far more skeptical by nature than Jim, also isn't certain that he can afford not to practice certain superstitious behaviors.
They are both looking to make a new and better life for themselves. Jim is running away in the hopes of finding freedom and a new life to which he can bring his family. Huck is running away to find his true identity, to escape from the life being forced on him by Pap and the Widow Douglas and everyone in his town.
One way that I think Jim and Huck are alike is that both characters desire freedom. Granted, they are seeking freedom from different things. Huck simply wants to escape from the routine and restrictions of society and school. Jim is seeking freedom from being a slave.
Both characters are also quite gullible. That's not necessarily bad, but it can be explained. Both characters are not highly educated, which explains why they are so gullible. Yet despite both being poorly educated, both characters have a strong moral compass. Both Huck and Jim know right from wrong and actively try to do the right thing. Both characters are kindhearted and honest as well. Lastly, both characters are incredibly loyal to one another. It is because of their multiple similarities that the two make such a wonderful and believable pair to read about.