The duke and the dauphin, also known as the king, are professional con men whom Huck meets later in the book. Although Huck catches on pretty quickly that these men are not, in fact, the royalty they claim to be, he is willing to tag along for a couple of reasons: he considers himself an orphan with nowhere else to really go and he has a difficult time turning away from a possible adventure. The con men travel from place to place with one scheming idea after another, and Huck is mostly a spectator in their antics. The men seem to prey especially on those with compassionate hearts. We see them visit what appears to be a religious revival meeting, telling the congregation that they are former pirates who want to take their "new" Christianity to their old pirate world. Their performance is so touching that the congregation gives them $87.75, which is the equivalent of around $2,700 in today's currency. When Huck finds a flyer for what appears to be Jim, his African American companion for much of his journey, the reader senses that this foreshadows upcoming tension between Huck and the con men:

It had a picture of a runaway . . . with a bundle on a stick over his shoulder, and "$200 reward" under it. The reading was all about Jim, and just described him to a dot. It said he run away from St. Jacques’ plantation, forty mile below New Orleans, last winter, and likely went north, and whoever would catch him and send him back he could have the reward and expenses.

Because Jim has become Huck's friend and because the con men seize every opportunity to cash in at others' expense, it seems clear that this is headed in a bad direction for Huck (and Jim). The con men do end up turning in Jim and using the reward for alcohol.

The con men continue to move from victim to victim, and Huck largely remains quiet until the two prey on the Wilks family. Huck is especially fond of Mary Jane and eventually turns on the con men, telling her the truth about their identities (and that they are not the uncles of the girls that they claim to be). This is a turning point in the story as the reader can see that Huck is maturing. He is breaking with adventure for the sake of adventure and making a choice to help someone else.

From the outset, Huck and Jim are ambivalent and somewhat distrustful regarding the King and the Duke. The wild lies that seem to leap from both men constantly are subtly recognized for what they are - lies - by both Huck and Jim. 

Even though he is aware of their true characters, Huck plays along...

For his part, Huck is caught in a (familiar) dilemma. He helps the King and the Duke in the first place because they quite simply were in need of help. Once they board the raft, they become part of Huck's "group", for lack of a better term. 

Huck's social code clearly suggests that betrayal is unacceptable. Abandonment is an infraction of the code as well. It is in this way that Huck finds himself in a position demanding his loyalty to these two crooks. 

Huck does not like the con men. He does not respect them, except for their exceptional abilities to dissemble and lie. In this area, Huck recognizes their virtuosity with astonishment. Yet, these are not skills that Huck truly admires, though he does recognize the ability to lie as a skill. 

Beholden to a social/moral code that compromises him, Huck cannot betray the King and Duke in good conscience. Additionally, he is worried that Jim may come to harm through the two men. 

This worry is a facet of the broad social-value system that places white males at the top of the social spectrum. Huck and Jim are socially less powerful than the King and the Duke and have been conditioned to respect, fear, or otherwise heed those above them in the social hierarchy.