Arthur "Boo" Radley is a character in the 1960/1962 novel/film To Kill A Mockingbird. He plays a very small role, but in the ending, he becomes much more important.
Radley is the subject of much prejudice from Maycomb. Many people believe that Radley is a horrible person, due to rumors spread about him and a trial he underwent as a teenager. Jem Finch and Dill Harris speculate that he is a criminal and are both afraid of him, but later in the book, Jem and Scout realize that Radley didn't come out of his house so long for a different reason: he doesn't want to. He doesn't like the outside world, full of prejudice and racism.
Near the ending of the film, after Scout and Jem are walking home from school in the nighttime, Bob Ewell, a person who was an enemy of their father Atticus Finch, almost managed to kill both Jem and Scout (intentionally). Although, Boo Radley rescued Jem (both children still alive), taking him back to Atticus' house safely, while Scout also ran to the house. Jem is hurt considerably more than Scout and is unconscious, so he does not know that Boo Radley was the one who saved him, but Scout does. Even when Radley is in a very positive light, he still is extremely nervous to the level that he cannot even say a word. Radley goes back home after Scout realizes who saved both Jem and herself.
Radley may be a very nervous person who has not been outside for many years, although he is also considerate and saves Atticus' children in the ending. He speaks only a singe line of dialogue, and his emotions are described with only his facial expressions, highlighting his nervousness to the outside world.