Passages in Harper Lee's To Kill a Mockingbird that relate Boo Radley, whose real name is Arthur Radley, to a mockingbird will help portray his innocence. As Miss Maudie explains to Scout, it's a sin to kill a mockingbird because they do nothing but sing for us all day long; they are innocent, unlike other birds who are guilty of destroying personal property.
While there is a rumor that Arthur, at the age of 30, stabbed his father with a pair of scissors and is now being kept prisoner in his own home as opposed to being sent to an asylum, Miss Maudie expresses Arthur's innocence. She points out that Arthur's father is excessively strict due to his very literal interpretation of the Bible. As Miss Maudie explains, Mr. Radley is a "foot-washing Baptist," and "foot-washers believe everything that's pleasure is a sin" (Ch. 5). Based on what Miss Maudie says, we can surmise that Arthur never leaves his house because he has been taught it is a sin to do anything pleasurable, and such teachings have made Arthur feel imprisoned. Hence, if Arthur is indeed guilty of stabbing his father, it would be because he has been driven insane by his father's strictness and beliefs. Being drive insane rids Arthur of any guilt.
Miss Maudie further paints Arthur's innocence when she remarks, "I remember Arthur Radley when he was a boy. He always spoke nicely to me, no matter what folks said he did" (Ch. 5). The fact that Arthur always speaks kindly to others shows that, like a mockingbird, he is not only innocent but also strives to bring others pleasure.
Arthur's desires to bring others pleasure is further captured in the gifts he begins leaving for Scout and Jem, such as leaving Jem's trousers neatly folded on the fence and mended and leaving things like balls of string, figurines carved out of soap, and chewing gum for the children to find in the knot hole of the tree on his property.
All of the above details help to portray Arthur Radley as innocent and desiring only to give others pleasure, just like a mockingbird.