The mockingbird is symbol for innocence: a symbol for one who does no harm to others, whose only purpose is to be helpful and peaceful. In Chapter 10, Scout recalls the time when Atticus told Jem to shoot all the bluejays he wants but that it is a sin to kill a mockingbird. Miss Maudie explains what Atticus meant to Scout:
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.
The characters most symbolizing mockingbirds in this novel are Tom Robinson and Arthur (Boo) Radley. Tom attempted to help Mayella and as a result, he was wrongfully accused, convicted, and killed. Tom only tried to help and never harmed anyone: Miss Maudie's definition of a mockingbird. By analogy, it was also a sin to kill Tom Robinson. Boo Radley also never did anyone any harm. In fact, there are numerous times in the novel when he helps Scout and Jem. Therefore, it would be a sin to harm or kill Boo as well. One might consider Atticus as symbolic of the mockingbird as well. He practices what he preaches: justice and honesty. He helps people in any way he can. And he even refrains from fighting back when Bob Ewell spits on him. Atticus generously reasons that to fight back would contribute to more violence:
So if spitting in my face and threatening me saved Mayella Ewell one extra beating, that’s something I’ll gladly take. He had to take it out on somebody and I’d rather it be me than that houseful of children out there. You understand? (Chapter 23)