Arthur, nickname Boo, is a man who is viewed as creepy from the neighborhood kids. The adults understand that he is just a shy person, but the kids don't see it that way. Jem, Scout, and Dill find him to be very creepy in the beginning of the story. The problem is that Boo hasn't had contact with the outside world for a very long time, making everyone uneasy of him. Due to an incident in his youth, his father forced him to keep quite and stay out of trouble inside the Radley household. He secretly leaves gifts for Jem and Scout in a hole in a tree in his yard because he is dying to make friends. Also, he saves Scout and Jem from the attack launched by Bob Ewell. In the end, the children see him for who he is, a shy man who turns out to be a hero.
Boo is one of the two "mockingbirds" in the story. He is a man who just wants to be free and escape his house, but he is trapped. Because of one mistake he made long ago, he is now suffering. Boo ends up showing both "moral courage" and "physical courage" when he saves the children from Bob.