The character of Arthur Radley, nicknamed “Boo,” is primarily revealed as very courageous late in the novel, when he saves the lives of the Finch children, Jem and Scout. In the process, he kills their attacker, Bob Ewell. However, these actions are the culmination of a process that builds throughout the novel. His courage is made evident earlier when he begins to emerge from his house, prompted by curiosity about the behavior of the Finches and their friend Dill.
Although the reader does not learn many of the facts surrounding Arthur Radley’s emotional or mental problems, by the time the novel’s action begins, he has apparently developed agoraphobia, the serious fear of going outside. It has been years since anyone in Maycomb other than his uncle Nathan has seen Arthur outside his home, where he lives with Nathan. Based on idle gossip and fueled by their overactive imaginations, the children become obsessed with “Boo,” and sneak into the Radleys’ yard to look inside the house. As Arthur realizes the children are interested but also benign, he makes gestures of friendship. One significant act of courage is his placing objects in an old tree, where the children find carved statuettes of themselves.
Although they have never spoken with Arthur, he is aware of their activities. Apparently aware that their father had been assaulted by Bob Ewell, who spit on Atticus, Arthur has been keeping an eye on them. On Halloween night as they walk home from school pageant, Bob Ewell attacks both children. Jem fights back to protect his sister who is both encumbered and protected by her bulky ham costume; Jem’s arm is broken in the scuffle. Later it is revealed that the person who intervened to stop the attack was Arthur; in the process he stabbed and killed Ewell.