Many of the names in the story are ironic – they seem to mean the opposite of what the characters really are. "Pesty" seems like a pest because she follows people around. But, unlike a pest, her presence is much appreciated. She always contributes a lot to whatever situation she's in, whether it's singing in the choir when she's at church or helping Mr. Pluto take care of the treasure when she's in his cave home.
Another example is the nickname Mr. Pluto. Pluto is the name of the Roman god of the Underworld. On the one hand this isn't ironic. The character Mr. Pluto does live in a cave, after all, and he is presiding over a massive treasure. But, ultimately Mr. Pluto is a kind man who has had a hard life, not a supernatural creature to be feared.
Similarly, the Smalls are anything but Small. They prove to be big in all the ways that matter – like caring and kindness. At the same time, they are small in the sense that that are humble, they don't think they are bigger than others.
The irony in the names of the characters helps put forth the story's theme of appearances, and how things get deeper and deeper the more we examine them. What do you think the name "Skinner" means? What about "Darrow"? Why do so many Darrows have the name "River"?
Pesty and Mr. Pluto are the characters most closely associated with clothing. Before church, Thomas is amazed that Mr. Pluto is wearing a really old, strangely shaped cloak "held with an enormous safety pin across his chest" (9.95). To complete the picture, he wears a "stovepipe hat" (9.95). Pesty's outfit is also unique: "She was dressed in pink tulle [thin shimmery fabric], and a blue, polka dot bonnet" (9.95). She also has on "white, high button shoes. They were the queerest shoes Thomas had ever seen" (9.95). It's easy to see that the duo's attire makes them stand out, building them up as unique characters.
Mr. Small gets particularly excited by their dress. He says, "A bonnet no less! And high buttoned shoes. The stovepipe house is knockout! History, I tell you. Out of time!" (9.96). He seems to think Mr. Pluto and Pesty look like characters from a history book, like people from another time. This builds up our image Pesty and Mr. Pluto as unusual characters.
The hat and shoes only make sense when we know about the treasure. Then we understand what Pesty means when she tells Thomas "Mr. Pluto got [the shoes] for me…" (9.107). She might be telling Thomas she got the shoes from the treasure. Remember, the treasure includes all kinds of beautiful things, including shoes and stovepipe hats. These items aren't just beautiful, but are also perfectly preserved antiques. Unless things were added to the cavern after Dies Drear's death (unlikely), they are at least a hundred years old.
The characters' occupations are big parts of their lives. For example, Mayhew is a professional actor. He's able to make his living acting in plays. But, he also uses the skills of his occupation to solve the real problems in his life. In fact, acting is Mayhew's solution to most problems. How does he hide his presence in town? By pretending to be Mr. Pluto. How does he stop the Darrows from tormenting the Smalls and Mr. Skinner? By putting on a play. How does he make friends with the Smalls? By involving them in acting.
Similarly, Mr. Small's love of history trickles into every aspect of his life. The whole reason the Smalls are living in the house of Dies Drear is because Mr. Small wants to study its history. His love of history is what makes him decide to help keep Mr. Pluto's secret. If he tells the foundation, he runs the risk of losing the chance to explore the treasure, which is valuable to him because of the historical stories it tells.