Tools of Characterization
While characters use clothing to express themselves and show their personal style (like Miss Edmunds and Leslie's preference for pants, and lack of makeup), they also use it to judge other characters and set up class standards. When Jess first meets Leslie, he can't even tell from the clothes she's wearing whether she's a girl or a boy. Think about Leslie's appearance on the first day of school, which clearly puts her in contrast with all the kids in Lark Creek:
Leslie was still dressed in the faded cutoffs and the blue undershirt. She had sneakers on her feet but no socks […] They [the other students] were all sitting there primly dressed in their spring Sunday best. (3.2)
Leslie doesn't appear super feminine or girly like the other kids in Lark Creek, but she also doesn't act like it either: she runs faster than any of the boys. That doesn't mean she's not a girl, though, or that she doesn't have dresses – she produces one to wear to church with the Aarons family. In addition, clothing can reveal a character's financial standing: Jess has that "worn-out" pair of sneakers, and his family doesn't even get new "spring Sunday best" (3.2) clothes when his father gets laid off work.
It's amazing that two such different families would produce such good friends like Leslie and Jess. Leslie comes in with every advantage: money, education, kindness, and attention. She's the only child of two forward-thinking artists who treat her like an adult and value her opinions. Leslie always knows she's special, yet she's also humble and forthright. In contrast, Jess has barely any advantages: he's the only boy with four sisters in a poor, undereducated family. His parents discourage his dreams of artistry, and he knows he's going to have to get wherever he's going by himself. Even though he doesn't feel special, he is.
Leslie opens Jess's mind to the possibility of whole new worlds, and Jess shares with Leslie compassion and practicality: she imagines a castle in Terabithia, and he helps build it. Jess observes that neither of them is or should be that comfortable with the other's family. And yet, in their shared alone time, in and out of Terabithia, he feels more confident and safe than he ever had before.
Speech and Dialogue
In Terabithia, Leslie and Jess talk differently than they do in Lark Creek, revealing the tremendous scope for imagination and creativity they both possess. Their language is formal, courtly, and kind. Terabithian language includes stuff like, "[we must go] to the grove of the pines […] this is a time of greatest joy" (6.19). It's almost spiritual, although there's humor in it too, like when they introduce P.T. to their secret world. Leslie's the instigator, and she teaches Jess to follow her. He learns to be a king, too:
[…] how a ruler must behave. That was the hard part. When Leslie spoke, the words rolling out so regally, you knew she was a proper queen. He could hardly manage English, much less the poetic language of a king. (4.107)
Jess may not feel like he can speak the Terabithian language as well as Leslie can, but he does know enough to silently correct his mother's grammar: when she says, "Oh, all right. But I ain't got no money to give you," Jess thinks, "Any money" (1.48-49). He's smarter than he gives himself credit for. At the end of the book, when he goes to the pine grove by himself, he is able to participate in the Terabithian language alone, saying, "It's a sign from the Spirits […] We made a worthy offering" (13.12).