Boo Radley is one of the mockingbirds of the story, an innocent. He has decayed as a result of his seclusion.
Throughout the story, there are references to mockingbirds. These are not literal birds, but people who are innocent and yet targeted by society. Scout says that the only time she ever heard her father Atticus say it was a sin to do something was when he says it is a sin to kill a mockingbird. When she asks Miss Maudie about it, she replies:
“Your father’s right,” she said. “Mockingbirds don’t do one thing but make music for us to enjoy. They don’t eat up people’s gardens, don’t nest in corncribs, they don’t do one thing but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird.” (chapter 10)
Boo Radley is one of the innocents in the novel who is victimized by his family and ostracized by society. The other significant mockingbird in the story is Tom Robinson, who is in fact killed when he is convicted of rape even though he is innocent, and he is shot attempting to escape prison.
This description of Boo shows how Boo has suffered from his isolation.
His cheeks were thin to hollowness; his mouth was wide; there were shallow, almost delicate indentations at his temples, and his gray eyes were so colorless I thought he was blind. His hair was dead and thin, almost feathery on top of his head. (chapter 29)
Because Boo never leaves his house, and has no contact with anyone, he has become shy and withdrawn. Even though he is relatively young, he is described as wasting away. This is what happens when an innocent is targeted by an unjust society.