Why does Lee introduce the character of Lula into the story?

Lula is angry at Calpurnia for bringing white children to a black church, "You ain't got no business bringing' white chillun here - they got their church, we got our'n."

Lee uses Lula as an example about how racism can work both ways. Lula dishes out the same racism that is inflicted on her. Racism does not belong to a particular group.