Although she is "just" the housekeeper to the Finch family, Calpurnia also serves as the surrogate mother to Jem and Scout. Since their mother died of a heart attack shortly after Scout's birth, Cal is the only adult female in the home. Atticus gives her free rein in disciplining and teaching the kids right from wrong--an unusual thing for a black women in a white home in Depression-era Alabama. When Aunt Alexandra arrives to take over as female head of the household at the beginning of the Tom Robinson trial, she tries to convince Atticus to fire Calpurnia, believing that she will no longer be needed. But Atticus knows differently: He stands up to his sister and declares firmly that Cal is a necessary and loved member of the family. Aside from teaching the children manners, she strictly rules the home and keeps them (especially Scout) on their toes. She also helps to educate them. It is Calpurnia who teaches Scout to write in cursive, an act that she regrets after Miss Caroline condemns her for not printing her work on the first day of first grade.
Calpurnia is as key as Atticus is to teaching Scout and Jem one of the most important lessons of their childhood—and therefore to letting the audience know the book's most essential theme: respect for others, particularly those who are different. Atticus's famous line "You never really understand a person...until you climb into his skin and walk around in it" is a lesson Cal routinely imparts to the kids—just not in those words. For example, when Walter Cunningham comes to lunch, Scout criticizes his behavior at the table. Cal lectures Scout to show hospitality to her guest but also to respect that other people have different backgrounds and behaviors.
Cal teaches this lesson again when she takes the kids to her church. Not only is she exposing them to a different community, but she also teaches them about showing respect for the differences in each community. Scout does not understand why Cal speaks differently with her church friends, but Cal explains that you show respect by adapting to the community you are in.
"Suppose you and Scout talked colored-folks' talk at home it'd be out of place, wouldn't it? Now what if I talked white-folks' talk at church, and with my neighbors? They'd think I was puttin' on airs to beat Moses."
"But Cal, you know better," I said.
"It's not necessary to tell all you know. It's not ladylike—in the second place, folks don't like to have somebody around knowin' more than they do. It aggravates 'em. You're not gonna change any of them by talkin' right, they've got to want to learn themselves, and when they don't want to learn there's nothing you can do but keep your mouth shut or talk their language" (12.139-144).