Boo Radley is introduced as a mysterious figure, possibly homicidal and definitely spooky. He is a local legend, and the neighborhood monster. The children have heard stories of how he attacked his parents with scissors or how he wanders the neighborhood at night eating children. None of the stories are true, of course. Boo is actually a quiet, shy, seriously abused young man. He does seem to have had a mental breakdown at some point, but he is gentle and not dangerous.
Boo Radley’s purpose in the story is to symbolize childhood, and the transition from childhood to adulthood. As the children grow and begin to experience more of the world, they learn more and more about Boo until the end of the book when they meet Boo face to face and learn the truth about him. Along the way, each interaction with Boo becomes more direct and coincides with their growth into adulthood. The presents in the tree, the pants left on the fence and saving Scout from Bob Ewell are all experiences with Boo that become more and more open. As the children grow and learn about the world, including the ugly parts like racism and prejudice, they grow to learn more about Boo (Arthur) Radley.