Who is Calpurnia in "To Kill a Mockingbird"? What is her place in the Finch household?

Aside from her roles as cook, housekeeper, and nanny, Calpurnia (who is African-American) is a kind of substitute mother figure for Scout and Jem.  Since their own mother has died, Calpurnia provides them with the kind of maternal guidance, supervision, love, and correction they might otherwise lack.  Just as Atticus is a kind of ideal father figure in this novel, so Calpurnia -- with her wisdom, common sense, and strength of character -- functions as a kind of ideal mother figure.  Certainly she is a more appealing maternal figure than many of the other women in the book, including the young school teacher and the bossy aunt (Atticus's sister).  As the scene in the African-American church indicates, Calpurnia is a representative of Christianity at its best.  In that scene and indeed throughout the novel, she displays such virtues as love, kindness, and acceptance.  However, she also displays the strength of character that helps to make her such an impressive and admirable figure throughout the novel.