Hank is the manager of a munitions factory in Connecticut, a man "nearly barren of sentiment" (0.5), but very smart and practical. In other words, the perfect person to take the stuffing out of all the thees and yea forsooths in King Arthur's court. He serves as the story's hero and protagonist, telling the tale in his own words and relating his adventures in the distant past.
What does Hank the Yank look like? The text never really says. Instead, we get a quick description of his personality right on the first page:
He attracted me by three things: his candid simplicity, his marvelous familiarity with ancient armor, and the restfulness of his company—for he did all the talking. (1.1)
That last bit is helpful, since it means he's going to spill out his tale for all of us to hear. As for the rest of his personality, we pretty much just pick it up as we go along.
Hank has a very strong sense of right and wrong, and he definitely believes in the American dream. Democracy? Good. Nobles flogging the peasants for no particular reason? Yeah, he has a problem with that. He says:
Some of those laws were too bad, altogether too bad… A gentleman could kill a free commoner, and pay for him—cash or garden-truck. (18.4)
His patriotism actually informs his sense of doing right: he's going to remake England with the American model, and in the process correct all of its injustices. He starts with his title, Boss, which he gets by popular vote: "I couldn't have felt really and satisfactorily fine and proud and set-up over any title except one that should come from the nation itself, the only legitimate source" (8.7). And from there, Hank the Yank jumps into a whole slew of grand ideas, from building factories and schools to a railroad.
Everything Hank implements is designed to eliminate slavery, raise men up from their ignorance, and generally spread truth, justice, and all that good stuff. He's pretty good at it too—not only does he get the job done, but it only takes him seven years:
A happy and prosperous country, and strangely altered. Schools everywhere, and several colleges; a number of pretty good newspapers… Slavery was dead and gone; all men were equal before the law; taxation had been equalized. (40.2)
How does he accomplish all of this? That brings us to the second big part of his personality: his engineering skills and general brainpower. Hank the Yank figures things out very quickly, honing in on the source of the problem. Just look at how he assesses the holy fountain:
The "fountain" was an ordinary well, it had been dug in the ordinary way, and stoned up in the ordinary way. There was no miracle about it. (22.5)
More importantly, he knows how to solve the problem very quickly, using easy-to-understand methods that anyone can comprehend. Consider the lightning rod he uses to destroy Merlin's tower. Hank supervises the work, successfully teaching medieval knights how to construct something they've never even conceived of before. In this way, Twain not only shows us how smart the Yankee is, but also how easily the epic nature of older Arthurian works can be debunked.
Hank's also an interesting hero because he doesn't grow and change a whole lot during the story. Most heroes display an arc, gaining wisdom, knowledge, and the occasional awesome magic sword over the course of the book. Not Hank, though. Sure he gets married, has a baby and scores himself a swell job as The Boss, but his wisdom and knowledge stay more or less the same throughout the story, and his outlook doesn't change much either.
Rather than being changed by his experiences, Hank tries to change the whole world. We get that early on—"In various quiet nooks and corners I had the beginnings of all sorts of industries under way" (10.1)—and it continues more or less unabated until the last chapters—"Consider the three years sped. Now look around on England. A happy and prosperous country, and strangely altered. Schools everywhere, and several colleges; a number of pretty good newspapers" (40.2). Hank faces a few challenges and adventures along the way, but he always dispatches them without much of a fuss… For a while anyway.
Though Hank the Yank does all kinds of amazing things, he ultimately fails at his grand schemes, thanks to the idiocy and superstition he works so hard to overcome. In the last chapters, Hank is condemned by the Church and his work is destroyed:
Our navy had suddenly and mysteriously disappeared! Also, as suddenly and as mysteriously, the railway and telegraph and telephone service ceased, the men all deserted, poles were cut down, the Church laid a ban upon the electric light! (42.9)
He fights back, but he has to destroy all of his factories so that the bad guys don't get their greasy mitts on them, saying:
I went out into the hills and uncovered and cut the secret wires which connected your bedroom with the wires that go to the dynamite deposits under all our vast factories, mills, workshops, magazines, etc. (42.10)
Then Merlin puts the nail in the coffin by sending Hank back to his own time and leaving his followers to die, a magical maneuver that renders Hank the Yank a tragic character at the end of the day.
Is this Twain's way of saying that even the best and brightest can't make a dent in human stupidity? Is it just a convenient way of explaining why we didn't actually have railroads and steamships in the Middle Ages? Either way, Hank gets to take it on the chin… and all of his brains and lofty goals don't amount to anything.