Arthur "Boo" Radley is a recluse. Not much is known about his parents which makes rumors and speculation circulate and exaggerate. Because Boo has chosen to live in solitude, he is of course mysterious, and he becomes fodder for the children's imaginations. They make up plays and stories, basically painting him as a monster. The irony is that he is, like Tom Robinson, a completely peaceful man that helps people without any expectation of a reward or even a thank you.
As Jem begins to learn about the hypocrisies of the adult world of Maycomb, he determines that Boo might not be odd at all:
I think I’m beginning to understand why Boo Radley’s stayed shut up in the house all this time… it’s because he wants to stay inside. (Chapter 23)
Boo, like Tom, is a mockingbird. Both men lead peaceful, simple lives and help others when they can. Boo and Tom symbolize what a mockingbird is as Atticus and Miss Maudie describe in the novel:
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)