BOO RADLEY

"Boo" Radley is the most mysterious character in To Kill a Mockingbird .

Maycomb children believe he is a horrible person, due to the rumors spread about him and a trial he underwent as a teenager. It is implied during the story that Boo is a very lonely man who attempts to reach out to Jem and Scout for love and friendship, for instance leaving them small gifts and figures in a tree stump. Scout finally meets him at the very end of the book, when he saves the children's lives. Scout describes him as being sickly white, with a thin mouth and hair and grey eyes, almost as if he was blind. During the same night, when Boo requests that Scout walk him back to the Radley house, Scout takes a moment to picture what it would be like to be Boo Radley. While standing on his porch, she realizes his "exile" inside his house is really not that lonely.

Boo Radley's heroics in protecting the children from Bob Ewell are covered up by Atticus, Sheriff Tate, and Scout. This can be read as a wise refusal of fame. As Tate notes, if word gets out that Boo killed Ewell, Boo would be inundated with gifts and visits, calamitous for him due to his quiet personality. The precocious Scout recognizes the danger: Renown would "kill the mockingbird." Boo Radley is a ghost that haunts the book yet manifests himself at just the right moments in just the right way. He is, arguably, the most potent character in the whole book and as such, inspires the other key characters to save him when he needs saving.

After the Tom Robinson trial, Jem and Scout have a different understanding of Boo Radley. “Scout, I think I'm beginning to understand something. I think I'm beginning to understand why Boo Radley's stayed shut up in the house all this time… it's because he wants to stay inside." 

Having seen a sample of the horrible things their fellow townspeople can do, choosing to stay out of the mess of humanity doesn’t seem like such a strange choice.

When Boo finally does come out, he has a good reason: Bob Ewell is trying to murder the Finch kids. No one sees what happens in the scuffle, but at the end of it, Ewell is dead and Boo carries an unconscious Jem to the Finch house. Finally faced with Boo, Scout doesn’t recognize him: after all, she’s never seen him before, except in her dreams. He is played by Robert Duvall in the movie. 
This was Mr. Duvall's first film role.