What point does Harper Lee want to make about courage through her use of characters in To Kill a Mockingbird?

In To Kill a Mockingbird, Harper Lee develops the theme concerning courage to show the difficulty and necessity of behaving courageously. She also intertwines her theme of courage with her themes concerning behaving like a gentleman and a lady to show that it takes a great deal of courage to treat others with the amount of respect required of gentlemanly and ladylike behavior, just as Atticus treats people.

Atticus is the primary character Lee uses to show the difficulty of behaving courageously, but many characters behave courageously throughout the book by acting in ways that are contrary to society's expectations. Atticus behaves courageously by putting his all into defending Tom Robinson, despite the rest of society's prejudiced belief that Robinson is guilty and does not deserve a defense simply because he is African American. More importantly, Atticus takes on this responsibility knowing the likelihood he will fail to acquit Robinson. Atticus defines courage as the ability to do what needs to be done despite the fact you are unlikely to succeed, as we see in his following speech to Jem:

I wanted you to see what real courage is, instead of getting the idea that courage is a man with a gun in his hand. It's when you know you're licked before you begin but you begin anyway and you see it through no matter what. (Ch. 12)

Beyond defining courage as doing what is right regardless of failure, Lee intertwines courage with gentlemanly and ladylike behavior in multiple places. Jem recognizes Atticus's courage when he learns Atticus opposes the rest of society by refusing to use his sharpshooting skills to hunt, because Atticus he prefers to be respectful towards all living creatures when the rest of society doesn't usually care. In treating all living creatures with respect, Atticus behaves gentlemanly, and since his behavior is contrary to the rest of society, Atticus shows that even behaving gentlemanly requires a great deal of courage, as Jem implies in his revelation once he understands why Atticus won't boast about his skills to easily kill living things: "Atticus is a gentleman, just like me!" (Ch. 10).

In addition, though Jem at first equates behaving like a girl with cowardice, he soon comes to realize that being a great lady, like Mrs. Dubose, requires the utmost bravery. Even Scout comes to equate being a lady with being courageous when, after Robinson's death, she sees Aunt Alexandra and Miss Maudie put on brave smiles and continue to treat their guests with the utmost respect when all they would like to do is dwell on their own worries and sorrows. Scout comes to realize that it takes a great deal of courage to behave selflessly in times of tragedy, as true ladies must.