It is first important to define what a mockingbird is. This comes out clearly in a conversation between Atticus and his children. Atticus says that it is a sin to kill a mockingbird. The children are shocked to hear Atticus's strong words. So, they ask Miss Maudie. Miss Maudie responds with these words:
“Your father’s right,” she said. “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.”
Based on this definition, a mockingbird is someone who always does good. So, if someone tried to hurt a mockingbird, then it would be a grave sin.
Jem, along with all the children, are mockingbirds. They are not only innocent, but they have good hearts. They want to do good. For example, Jem invites Walter Cunningham for a meal, when he and Scout were fighting. Jem also reads to Mrs. Dubose in her illness. Atticus also says that if the jury were comprised of boys like Jem, Tom Robinson would be free. Finally, at the end of the book, Jem seeks to save Scout from Bob Ewell's mad attack.
Mockingbirds remain the constant symbol of innocence throughout the book, and Miss Maudie explains to Scout exactly why:
"Remember it’s a sin to kill a mockingbird.” That was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it. “Your father’s right,” she said. “Mockingbirds don’t do one thing but make music for us to enjoy . . . but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird.” - Chapter 10
Though there are many symbolic people who are equivalent to mockingbirds in the story, Jem remains one of the people most affected by the events of the novel. He grows over the course of the novel, and his awareness of the evil inflicted on Tom Robinson snatches Jem's innocence away from him. Suddenly he is forced to confront the racism, injustice, and poverty that revolves around him in his town of Maycomb. Because of these events, Jem becomes a mockingbird injured by the world that surrounds him.