What characters best exemplify courage, and what point does Harper Lee want to make about courage through her use of these characters in To Kill a...

Using Atticus Finch's definition of courage-- 

"It's when you know you're licked before you begin, but you begin anyway and you see it through no matter what"--

Mrs. Dubose, of whom Atticus speaks in this quote, is courageous, and, of course, Atticus is also courageous, as well as Boo Radley.

• Mrs. Dubose

Jem is assigned to read to Mrs. Dubose after he angrily destroys her camellias because she has insulted his father by saying,

"Your father's no better than the n****rs and trash he works for."

During these visits, Mrs. Dubose lies in a semi-conscious state as Jem reads. But sometimes she utters "blood-curdling inventions." It is not until after her death that Jem learns that Mrs. Dubose has been a morphine addict; however, before she passed away, she bravely decided to die aware of the world in her last moments by withdrawing from this powerful drug. Atticus tells Jem,

"According to her views, she died beholden to nothing and nobody. She was the bravest person I ever knew."

• Atticus Finch

Knowing that it will certainly be surprising if Tom Robinson is declared innocent of the charges brought against him by Bob and Mayella Ewell, Atticus takes the case and becomes the attorney for the defense in Tom's trial. Atticus is brave because he also knows he is "licked" before he begins, but he begins anyway and "sees it through no matter what."

Atticus takes this brave move as he feels he must. He explains to his daughter Scout that Tom Robinson's case goes to "the essence of a man's conscience." Further, he allows others their opinions, but he says that he must live with himself. For, "[T]he one thing that doesn't abide by majority rule is a person's conscience."
And, so, he tells his children to be prepared for vituperation, but not to lash back. Atticus bravely protects Tom at the jailhouse and endures the insults and affronts by many in community before and after the trial. 

• Boo Radley

When the reclusive Boo hears the frightened cry of Jem as he is attacked by Bob Ewell, Arthur Radley bravely rushes out of the house which he hardly ever leaves. He rushes Ewell and saves the lives of Jem and Scout by wrestling Bob's knife from him. Without question, he risks his life to save the Finch children.