How is Aunt Alexandra important to Harper Lee's novel To Kill A Mockingbird?

Aunt Alexandra is important to the novel's themes of class attitudes and traditions, prejudice, and the instructional design of this bildungsroman. She also acts sometimes as a foil to Atticus.

• Class attitudes and traditions and social prejudice

In Chapter 13, when Aunt Alexandra arrives at the Finch house purportedly to assist her brother who is busy preparing for the controversial trial of Tom Robinson, she actually injects more stress into the lives of all those in the home. For one thing, she advises Atticus with matriarchal authority that the children must be made aware of their social position in Maycomb. So, Atticus uncomfortably tells Jem and Scout that their aunt wants him

"...to try and impress upon you...that you are not from run-of-the-mill people, that you are the product of several generations' gentle breeding--"

This implies that Scout must wear dresses and behave with more appropriateness to her station in life, a criticism much like those of Mrs. Dubose's.

• Racial prejudice

When the "pecking order" is disrupted by Aunt Alexandra's overriding Atticus's authority by telling Scout that she may not accompany Calpurnia to church again, Scout tells her father, "I didn't ask her, I asked you--"  Atticus tries to solve the issue by telling Scout to obey whoever tells her to do something. And, Alexandra tells Atticus that he must "do something about her." Then, she tells Atticus to fire Calpurnia because they do not "need her anymore," implying that the maid is actually a bad influence on Scout as Scout should not be in a black church, nor be dependent upon Calpurnia for motherly advice.

Further, although Alexandra really does not approve of Atticus's being the defense attorney for Tom Robinson, she is angered when her guests rebuke him. Further, when Mrs. Merriweather disparages her black maid Sophy, but lauds a missionary from her church who works with Africans, Alexandra is relieved when Miss Maudie makes a cynical remark which obviously is directed at Mrs. Merriweather's racial hypocrisy. 

• Maturing motif (bildungsroman)

From her experiences on the day of her Missionary Tea, Alexandra begins to gain a wider perspective of current events. Certainly, she becomes very concerned for her brother Atticus because the trial is emotionally trying for him. Even though she may not agree philosophically with her brother, she certainly gains insight into the harm caused by biases, moral weakness, and prejudice, no matter to whom they are directed. Alexandra remarks to Miss Maudie,

"It tears him [Atticus] to pieces....what else do they want from him, Maudie, what else?....They're perfectly willing to let him do what they're too afraid to do themselves--it might lose 'em a nickel."

Miss Maudie calmly advises her to be quiet as the ladies may hear her; then she adds that Maycomb is, nevertheless, paying Atticus "the highest tribute" since it trusts him "to do right" when they are afraid to do it themselves. Finally, Miss Maudie suggests that they should join the ladies in the parlor. Pulling herself together, Aunt Alexandra nods at Scout who rises to the occasion by carefully picking up a tray and stepping into the parlor to serve the ladies. Scout narrates, "After all, if Aunty could be a lady at a time like this, so could I."