Consider how Jem, Scout, Boo and Tom are all mockingbirds: innocents with injustices inflected upon them in To Kill a Mockingbird.
Tom Robinson is obviously innocent. His left arm is dysfunctional, and Mayella was beaten by someone who led exclusively with their left. And who shoots someone 17 times? Tom's death truly embodies the fact of his innocence in trying to escape, because a good guard would have shot to wound, not kill.
Jem is an interesting character of innocence. We watch Jem move from childhood to puberty throughout the novel but most specifically during the trial. By trial time, Jem was old enough to understand the evidence and make a clear and obvious decision regarding Tom. However, his childhood innocence was almost destroyed through this process as he saw the evil in men's heart arise in the verdict. It took him several days to process this. Because he wouldn't just accept this at face value and allow racism to become a part of him, Jem is innocent.
Scout's childhood innocence is displayed throughout the novel in her naive approach to school, Mrs. Dubose, her father's abilities, Dolphus Raymond, the trial and the things women say at the Missionary tea. What rocks about Scout is that although she sees all of this throughout the novel she sticks to the idea that there is one kind of folks. Folks. I think this novel would have been entirely different and of less impact had Scout been a teenager.
Lastly, Boo is allegedly a terrible scary phantom who eats squirrels and cats while he spies on Stephanie Crawford. We couldn't be further from the truth. By the end, we truly see he didn't deserve any of these neighborhood allegations as he made the choice to save the kids' lives even if it put his own at risk.