Hank is Twain's idea of the quintessential American: inventive, ambitious, and fiercely egalitarian. He inspires major changes in Arthurian England, but his innovations are all destroyed by the Church and the war following Arthur's death.
Clarence is Hank's first friend in the 6th century. When Hank is imprisoned, Clarence delivers messages for him to the king and the court. Once Hank is freed and named second most powerful man in the kingdom, he makes Clarence his top assistant. Clarence is one of the characters who give Hank insight into the 6th-century mindset. Although Clarence is trained to think like Hank, he still understands and, in some ways, sympathizes with some of the beliefs and attitudes of the common peasants.
Unlike Clarence, Sandy never really learns to think like Hank, and Hank doesn't expect her to do so. She does serve as another source of insight into the 6th century, and in fact she helps Hank manage some of the intricacies of chivalrous behavior. She is devoted to Hank. He suggests he only marries Sandy because he is afraid she will be "compromised" by hanging around him so much. But over the years he grows deeply devoted to Sandy and is heartbroken to lose her when he goes back to the 19th century.
King Arthur is a legendary figure who is meant to represent nobility, chivalry, and honor. Twain presents Arthur as noble but naive, well intentioned but unaware of what life is really like in his kingdom. Nevertheless, Hank develops a genuine affection for Arthur and is saddened when Guenever betrays Arthur and the civil war leads to Arthur's death.
In Arthurian legend, Merlin is a magical advisor who helped Arthur ascend to the throne, acquire the sword Excalibur, and rule the country. In Twain's version, Merlin truly believes in his own magic, but he is incapable of really accomplishing anything. Hank has disdain for Merlin, but allows him to continue doing "harmless" bits of magic. Merlin fiercely resents Hank for showing him up as a magician and allies himself with anyone who opposes Hank. He tries to help Sir Sagramor defeat Hank in a duel, and when Hank leads his "boys" in a war against the knights, Merlin disguises himself and sneaks into Hank's refuge to get revenge. Twain is unclear on exactly what Merlin does to Hank—Merlin claims it is a spell—but he somehow sends Hank back to the 19th century.