The mockingbird symbolizes innocence and vulnerability. Numerous characters lose their innocence in the novel, for instance, Tom Robinson loses his innocence when he is killed, and the newspapers compare his death to the meaningless killing of a songbird; which refers to Atticus' warning to his children about killing a Mockingbird as a sin because a mockingbird does nothing but bring joy to people. In addition to that, the sheriff dodges a situation of having to arrest Boo Radley because it would be like "shooting a Mockingbird." Boo is very vulnerable and subjecting him to more hardship would not be fair. Scout's last name Finch, another songbird, suggests that her innocence is very important to the plot of the story.

The characters with physical challenges in the story include Tom Robinson whose left hand is mangled; Jem breaks his hand badly that the damage remains permanent; Boo Radley has very low self-esteem, and Atticus has poor eyesight. These physical challenges are a symbol that everyone has a weakness which represents the damages brought about by situations in life.

As a way to connect to the outside world as he has always wished, Boo leaves mysterious gifts to Jem and Scout in the knothole. This is his way of connecting to people without exposing himself to any danger. When Nathan Radley learns of Boo's tactic, he fills the knothole with cement to keep Boo away from the outside world.

Tim Johnson, a popular dog in Maycomb, falls sick and becomes a danger to the community. The dog's illness is used to symbolize racism in the town. The same way sheriff refused to bring justice in Tom Robinson's case, he refuses to shoot the dog and encourages Atticus to fight it just the same way he fought for justice in the courtroom. Miss Maudie tells Scout that she believes that Atticus “decided he wouldn't shoot till he had to, and he had to today.”