This is a great question. Calpurnia is a black woman who takes care of the Finch house. As such, Calpurnia represents the black community in Maycomb, not perfectly but sufficiently.
First, Calpurnia is an honest woman who really cares for Jem and Scout. She makes the Finch household functional. Without her, things would fall apart. Atticus not just admits this; he clearly says this in a conversation with his sister, Alexandra. Here is what he says:
Atticus’s voice was even: “Alexandra, Calpurnia’s not leaving this house until she wants to. You may think otherwise, but I couldn’t have got along without her all these years. She’s a faithful member of this family and you’ll simply have to accept things the way they are."
This says volumes about Calpurnia. She is family. Atticus also says that Calpurnia is just and a good disciplinarian. Listen to what he says:
“Besides, I don’t think the children’ve suffered one bit from her having brought them up. If anything, she’s been harder on them in some ways than a mother would have been... she’s never let them get away with anything, she’s never indulged them the way most colored nurses do. She tried to bring them up according to her lights, and Cal’s lights are pretty good—and another thing, the children love her.”
Another important insight about Calpurnia comes in chapter 12, when she takes the children to her church. When Lula, a black congregant, opposes her for bringing Jem and Scout to church, Calpurnia defends them. Moreover, she says that they worship the same God. This shows that even though there are a few racist black people, there are many non-racist ones as well. Furthermore, the church experience shows that the black community is far more sincere in their Christian faith than Mrs. Merriweather and her missionary society friends.
In a touching scene, Rev. Sykes starts a collection for Tom Robinson. Even though the black community is poor, they give sacrificially. Here is another quote:
He read from another paper. “You all know of Brother Tom Robinson’s trouble. He has been a faithful member of First Purchase since he was a boy. The collection taken up today and for the next three Sundays will go to Helen—his wife, to help her out at home."