For all his internal mucking about with matters of conscience, Huck really comes down to his actions. For example, he risks getting caught to try to save the robbers; he reveals the inheritance hoax to Mary Jane; and he notably does NOT send a letter to Miss Watson ratting Jim out. Jim is also defined by his actions. He stays up nights to take Huck’s shifts at watch, and more importantly he comes out of hiding at the end of the novel to save Tom’s life. The duke and the king, of course, are who they are because of their cons—their immoral actions against those they meet.
Check out the foil section where we talk about Buck and Huck. It’s no coincidence that their names are essentially the same, which means Twain is using names to give us information. Then think about the duke and the king: they’re both con artists, and even their names reflect 1) their constant deception of those around them, and 2) their delusional belief that they are somehow high-class, important people.
Of course, Huck wouldn’t be Huck without all those great internal moral crises, which thanks to his first-person narration aren’t so much internal as they are directly in our face. Then you’ve got all his conversations with Jim—we’re thinking in particular of the "Solomon was a jerk" and "French is stupid" moments. Through their thoughts and opinions, we are able to recognize Huck as a conscientious, good-hearted kid, and Jim as a logical and intelligent man.
As Twain remarked in the second of the two opening notes, the dialects in Huck Finn are intentionally varied. This is pretty easy to see even with a basic comparison between Huck’s words and Jim’s. Huck’s vernacular is filled with colloquialisms and altered words—"that ain’t no matter," "it warn’t no time to be sentimentering," etc. Still, with all these liberties, the words are highly recognizable, easy to read, and devoid of a heavy accent.
Jim’s speech, we see, is markedly different: "I ain’ gwyne to len’ no mo’ money ’dout I see security. Boun’ to git yo’ money back a hund’d times, de preacher says! Ef I could git de ten cents back, I’d call it squah, en be glad er de chanst." Words are truncated or smushed together, and we hear much more of an accent than we do from Huck. So Twain wasn’t kidding when he said he did a careful job of giving everyone his or her own manner of speech.
The duke and the king are all about those sophisticated and high-fallutin’ words. Unfortunately, they get most of them wrong. The classic (and hilarious) example is the use of "orgies" as a substitute for "obsequies." With all their butchered fancy-shmancy talk, the duke and king make it clear that, in fact, they’re nothing but two uneducated, immoral rubes who have dressed up in fancy clothing and given themselves fake royal titles.