Calpurnia is officially the Finch family's housekeeper, but she also serves as Jem's and Scout's surrogate mother, and there is no doubt that Atticus keeps her on because of her female presence. Atticus also considers her "a faithful member of this family."
She had been with us ever since Jem was born, and I had felt her tyrannical presence as long as I could remember.
Calpurnia does the cooking (Scout loves her crackling bread) and cleaning, and she serves as the children's babysitter when Atticus is not present. Cal has a home of her own and leaves the Finches each evening, returning in the morning; sometimes she stays overnight, sleeping on a cot in the kitchen. She has taught Scout how to write cursive, and she keeps a watchful eye over her practice of penmanship. Cal takes Jem and Scout to her church one Sunday when Atticus is out of town, and she reports to Atticus on the day of the trial to announce that the children are missing.
When Aunt Alexandra comes to live, she attempts to force Atticus to fire Calpurnia, believing that she is no longer needed. But Atticus will hear none of it, telling his sister that
"Calpurnia's not leaving this house until she wants to. You may think otherwise, but I couldn't have gotten along without her all these years...
"I don't think the children have 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, and she's never indulged them...-- and another thing, the children love her."