Boo Radley is an innocent by virtue of his lack of contact with society. He has been "shut away" from mainstream society for a number of years and has virtually no contact with anyone else.
It may be that Arthur Radley's mental status is not 100% creating a situation in that he is forever a child without the adult ability to reason. However, Boo does know right from wrong and makes a heroic effort to save the Finch children from the vicious attack made on them by Bob Ewell.
Arthur "Boo" Radley serves as a fearful, mythologized presence through the majority of To Kill a Mockingbird, despite never actually appearing until the end of the book. Boo's habits and appearance become the subject of urban legend after he is placed under house arrest as a young person; his reputation, fueled by town speculation and gossip, becomes that of a crazed maniac who prowls the streets of Maycomb at night and eats raw cats and squirrels. These stories are expanded by the elaborate games of Scout, Jem, and Dill, who re-enact the life and times of Boo in an attempt to get him to come outside.
Despite all of these tall tales, Boo is actually a very innocent person and is one of the metaphorical mockingbirds referenced by the book's title. Although he is a good person, he is threatened by the presence of evil. Rather than being the monster that the town seems to think he is, Boo is a passive, gentle creature. He leaves chewing gum in the tree for the children and performs various acts of secret compassion, from placing a blanket on Scout after the fire to rescuing Scout and Jem from the violent clutches of Bob Ewell on Halloween.