An author uses other characters in a story to move in a new direction or to explain something in a story or give information about one of the other main characters:

Dill Haris-  Dill has an outsider's status in Maycomb. While Scout and Jem know nothing except Maycomb, Dill brings a sense of mystery and the world outside Maycomb's boarders. He also acts as a mediator between Dill and Scout. 

Maudie Atkinson- Miss Maudie represents a femenine counterpart to Atticus. Although she never went to law school, she posesses a keen wit that leaves the bigoted class conscious women of Maycomb speechless. 

Aunt Alexandra- To an extent, Aunt Alexandra is a character foil to Atticus. Her prejudices and southern beliefs run contrary to Atticus's very enlightened beliefs. I think Aunt Alexandra, like many white characters, is a product of her environment. Atticus,however, holds powerful meanings in the humble way he goes about saying and doing things. Certainly, by the end of the book, Aunt Alexandra's prejudices about social status are tempered by her brother's heroism and tragic loss of Tom Robinson.