Calpurnia is the housekeepr in the Finch household, She also serves as mother figure to the two Finch children Scout and Jem. Calpurnia teaches the children manners and how youngsters are expected to behave. She also teaches the children about life for a black person during the time, the children have an opportunity to see how the blacks live when Calpurnia takes them to church with her. Atticus shows his trust in Calpurnia when he has her spend the night with the children when he feels they are in danger. Calpurnia turns out to be much more than just a housekeeper for the Finch household.
Calpurnia is the Finches housekeeper, and her primary role in the novel is to be a surrogate mother for Scout and Jem.
Early on, Scout is antagonistic toward Cal, as the two clash over Scout's treatment of guests. When Walter Cunningham, Jr. comes for dinner and pours syrup on his plate, Scout calls out the offense, much to Cal's chagrin. Cal teaches her to honor the guest - host relationship, that a host must make a guest feel welcome despite differences in peculiar habits.
Later, Cal will have a great impact on Scout and Jem's worldview when she takes them to the First Purchase Church. This opens the children's eyes in terms of the impact of Jim Crow laws on the black community in the deep South. More, the experience shows the solidarity among the poor blacks, for they raise money for the Robinson family. Cal imparts advice similar to Atticus': you don't have to tell everything you know. This reinforces the lessons empathy and restraint in communicating with others.
Still later on, when the kids become targets of violence, Atticus will ask Cal to stay the night with the kids for protection. Such is his trust of Calpurnia.