I need to cast my vote for Calpurnia here. Possibly as much as Atticus, I believe that she, in becoming "almost" one of the family, has had as much (if not more) of an effect on Scout in combating racism as her own father does. The irony is that where Atticus makes his humanitarianism known through defending Tom Robinson and other glorious actions (like his stand-up a the jail), ... Calpurnia makes hers known in smaller ways: tender care for Atticus' kiddos, ... a seat for the two at church and in the courtroom, ... her tiny comments about equality peppering the text. Further, Calpurnia simply existing as a truly good, caring, ethical, just, tenderharted member of a race other than Scout's own must make a huge impression on her!
Father Atticus is surely the most influential character for both Jem and Scout in To Kill a Mockingbird, but Miss Maudie Atkinson offers Scout a maternal source of knowledge and guidance throughout the novel. Unlike many of the gossipy women of Maycomb who spout racism and second-hand information, Maudie (like Atticus) answers Scout's questions truthfully and without ulterior motives. Scout considers her "our friend," a neighbor who does not pry into their lives or play "cat-and-mouse with us." When Scout asks about Boo, Maudie gives her none of the speculative tall tales heard by Miss Stephanie; instead, she tells Scout that Boo is alive and "always spoke nicely to me." Maudie is a true friend of the Finch family, and she always supports Atticus. Scout recognizes her loyalty, especially when Maudies speaks harshly to Mrs. Merriweather after her veiled comments about Atticus. Like Atticus, Maudie leads by example, and Scout is impressed with her composure and ladylike ways at the missionary tea after learning of Tom's death. Scout wonders how Maudie can seem so unconcerned about her burning house, and she marvels at the positive outlook she has afterward. Scout seeks out Maudie's approval of her Halloween costume, and wonders why Maudie is not eligible to serve on a jury. She probably wishes that Maudie would accept her Uncle Jack's joking proposals of marriage, because then she could be called Aunt Maudie--a character worthy of inheriting the Finch name.
Atticus is the most important character in the novel, because he symbolizes the integrity that is sorely lacking in Scout's town. He is persecuted for his stance and that allows readers to understand why people find it hard to stand against injustice.
Another important, though minor, character is Walter Cunningham. He helps the reader and Scout see why the people in the town are so hateful, small-minded and racist. His grinding poverty, pride and anger will turn him into a bigoted, murderous adult.
With these two examples, Lee attempts to explore the motivation of victims and perpetrators alike.
However, I have always found it interesting that a supposedly anti-racist novel pays so little attention to its one major black character, Calpurnia.