In the novel, Attics explains to Jem that it is a sin to kill a mockingbird even if he shoots other birds. Specifically he says, "'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." Miss Maudie supports Atticus' rule and says, “Your father’s right,” ... “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.”
These lines help the reader to see the mockingbird as a symbol of innocence, almost a frail innocence, that should not be tarnished or dirtied by outside influence or evil. They are simply there to provide things for others to enjoy.
Thinking along those lines, characters in the story who display this giving and innocent nature are Boo Radley, Tom Robinson, and Dolphus Raymond could be considered mockingbirds. Boo is attacked for his differences based upon gossip and rumor, but really he only provided what he could for his neighbors. Tom did the same for Mayella Ewell and is eventually persecuted for it, and Dolphus was also harshly judged for his forward thinking lifestyle choices.