Lula appears only once in the novel, but she makes it count. When Calpurnia takes Jem and Scout to the African-American First Purchase Church, Lula is the one person who doesn't roll out the welcome mat for the Finch kids. In fact, Lula is the lone black voice of anything approaching anger.
And you know what? She has a point. The Finches are invading a sanctuary that has been painfully carved out of a white realm in which African-Americans are second-class citizens. Lula's momentary flare-up reminds us that Maycomb's black community could be approaching this whole ugly debacle very differently, and it makes us ask why she's the only one who doesn't appear quietly resigned.
Are the others concerned about the very real dangers that any behavior seen as threatening would bring down on their heads? Or is the novel itself just committed to showing black people as harmless and docile? It's hard to say from Lula's brief appearance, but she does make the novel's portrayal of African-Americans more complicated.