In Harper Lee's To Kill a Mockingbird, there are several characters that show courage. Atticus is definitely one, standing up for what he believes in defending Tom Robinson without thinking that Tom is a black man—he is simply a man deserving a fair trial.
The second character who comes to mind is Boo Radley. We learn early on that Boo (Arthur) got into a little trouble as a teenager. Where the other boys involved were sent away to a school as punishment (where they could straighten themselves out and learn a trade to help them become responsible citizens), Boo is at first locked up at his family's wishes in the jail—until the authorities say they can no longer keep Boo there: he must go home. At this point, Boo's father (a man of strong religious beliefs and a practice of delivering unreasonable physical, mental and emotional punishment) locks Boo in the house for years until the rumors fly and Boo is no longer young. We know he roams about at night, but he is never permitted (and finally probably does not wish) to come outside even after his father dies. Then Boo's brother, Nathan, becomes Boo's "keeper."
So Boo has no friends and never sees anyone, but we know he watches the children. He leaves gifts for them in the tree; he laughs inside the house when Scout's tire (with Scout) rolls into the side of his house, dumping the dizzy little girl on the ground. Boo puts a blanket around Scout's shoulders the night Miss Maudie's house burns, and sews and folds Jem's pants when they get caught on the fence in the Radleys' yard.
At the end of the story, Boo bravely battles Bob Ewell (certainly as stronger and healthier man than Boo) to save Jem and Scout's lives.
The two quotes that point to Boo's bravery are as follows. At the end of Chapter Twenty-Nine, Scout sees the man who carried the injured Jem back to the house. We can infer that this person is scared: he is pressed up against the wall of Jem's room, and he is shaking. Scout writes:
When I pointed to him his palms slipped slightly, leaving greasy sweat streaks on the wall, and he hooked his thumbs in his belt. A strange spasm shook him, as if he heard fingernails scrape slate, but as I gazed at him in wonder the tension slowly drained from his face, his lips parted into a timid smile, and our neighbor's image blurred with my sudden tears.
"Hey, Boo," I said.
The eNotes analysis of this chapter notes:
For Boo, entering society is a powerful act of bravery.
The second quote comes in Chapter Thirty, from Sheriff Heck Tate who is trying to protect Boo from the notoriety that would follow if folks in Maycomb learned of Boo's heroic efforts. Tate says:
I never heard tell that it's against the law for a citizen to do his utmost to prevent a crime from being committed, which is exactly what he did, but maybe you'll say it's my duty to tell the town all about it and not hush it up...To my way of thinkin', Mr. Finch, taking the one man who's done you and this town a great service an' draggin' him with his shy ways into the limelight—to me, that's a sin.
We can infer, with Heck Tate's statement that being a hero for this "shy man" would be a horrible experience; Boo is brave because he did what needed to be done to save the children, regardless of what it meant to him personally. He would hate to be hounded by people and stared at. Boo lives in the shadows. This, too, supports the sense that Boo Radley is a courageous, though unlikely, hero.