Let me start off with some background information.
Maycomb, where the novel takes place, is a sleepy town where nothing ever happens. In this context, the Radleys are seen as outsiders. They live in a run-down place and Arthur "Boo" Radley never comes out. The people of the town know that the Radleys have issues. As a boy Boo got in trouble with the law so his father locked him in the house. When Boo was fed up, he stabbed his father with scissors.
The children have a very vivid imagination when it comes to Boo. There are stories about him. For example, Jem says:
Boo was about six-and-a-half feet tall, judging from his tracks; he dined on raw squirrels and any cats he could catch, that's why his hands were bloodstained—if you ate an animal raw, you could never wash the blood off. There was a long jagged scar that ran across his face; what teeth he had were yellow and rotten; his eyes popped, and he drooled most of the time.
At the end of the novel when people found out that Boo was actually the one who saved Scout from Mr. Ewell, they realized that Boo was not a creepy person but a guardian angel. The reader also comes to this conclusion, because the first part of the novel leads the reader to think that the children's description of Boo might not be far off the mark. At the end of the novel, the truth about Boo comes out.
In light of this, Boo is redeemed in the eyes of all. He is a kind person, even though he is still a recluse. From a literary point of view, he is a mockingbird - one who does no harm to others. Nate sees this as do Atticus and Scout. There is much more than meets the eye when it comes to Boo and the Radleys.
Finally, I will add a few links on the novel.