The rat in this instance is named Templeton. He is the rat who helps Charlotte and Wilbur only when offered food. He serves as a somewhat caustic, self-serving comic relief to the plot. Avery Arable is the elder brother of Fern. Like Templeton, he is a source of comic relief.

Basically, in this novel, it tells the story of a pig named Wilbur and his friendship with a barn spider named Charlotte. When Wilbur is in danger of being slaughtered by the farmer, Charlotte writes messages praising Wilbur (such as "Some Pig") in her web in order to persuade the farmer to let him live. The only human in the story capable of understanding nonhuman conversation is Fern Arable the farmer daughter.

More Info: en.wikipedia.org