First, Cal fills the role of a mother figure for Jem and Scout. While Scout and Cal may clash at times, Cal is deeply protective of Scout and loves her dearly.
Cal also reinforces one of the main themes of the novel - to crawl inside someone else's skin before you judge them. This is evident early in the novel when Scout has Walter Cunningham Jr. over for lunch. He pours syrup all over his lunch. Scout is shocked and makes Walter feel foolish. Cal calls her into the kitchen and tells her that she has no right to shame a guest. Here Scout begins to see that her view of the world is not the only one, for Walter never has syrup to put on any of his meals, so when he is offered the chance, he takes it.
Cal also serves to represent another key theme - race. This is evident when Cal takes Jem and Scout to her church. Here Scout learns more of the background concerning the Tom Robinson trial. She also sees how the African American community has come together to support Tom and his family. Here too Scout sees that Cal resides in two different worlds (much like Scout herself, for she is straddling the worlds of youth and adulthood and childhood and womanhood). In the 'white' world, Cal is their educated mother-figure. In the 'black' world, Cal is a mother to her own children and a member of the church. This latter world is one Jem and Scout really never knew existed.
Calpurnia, much like Atticus, stands up against racism. She takes Jem and Scout to the African American church where they feel, for the first time, what racism must be like as the congregation is unhappy about Calpurnia's decision to bring white folks to church. What makes this incident even more important is that it illustrates that racism and prejudice cross color lines; that is, paradoxically, the Blacks are prejudiced against the Whites, and this takes place in a religious setting, where one should love his neighbor, no matter the color of skin. Calpurnia also acts as a surrogate mother to the children in the motherless household. This illuminates, perhaps, the reasons why Scout acts the way she does and seems to have problems with other female characters in the book.
She is very caring . She gives Scout and Jem lots of love and respect. When Scout and Jem where playing outside in the hot summer day with Dill, Calpurnia went out and gave them all lemonade. Scout said that she always did stuff like that. Calpurnia also likes to have them around. When Scout and Jem weren’t with her for most of the day she said she missed them and the house wasn’t the same without them. She also provides company for Scout when Jem is avoiding her.