Several characters in the novel resonate with the symbol of the mockingbird.
It is Miss Maudie who affirms that it is a sin to kill a mockingbird, since "they don't do one thing but sing their hearts out for us."
If we take the mockingbird to symbolize harmlessness and some measure of innocence, we can identify at least four characters as "mockingbirds".
Boo Radley and Tom Robinson are often discussed in this light. Radley only performs acts of kindness in the novel, though he is publicly conceived of as a freak. He gives secret gifts to Jem and Scout then saves their lives. Tom Robinson helps Mayella Ewell out of a sense of kindness and is convicted falsely of rape as a consequence.
These characters are harmless and need protection, like mockingbirds, yet they are not the only characters that fit this description.
Atticus Finch has a touch of innocence about him as well. He trusts in the better nature of his enemy, Bob Ewell, assuming that Ewell is finished with his revenge when he spits in Atticus' face on the street. Atticus does not believe that he and his children need protection too.
His innocence is demonstrated on this point when Ewell attacks Jem and Scout. It is demonstrated immediately afterward as well in his coversation with Heck Tate about how Ewell dies. In his idealism, Atticus is harmless, helpful, and innocent like a mockingbird.
The children, Jem, Scout and Dill are each also like mockingbirds. They need protection. Though Scout articulates an urge to protect Boo Radley at the novel's end, she could just as well be speaking about herself, her friend and her brother.
Scout tells her father that revealing Boo Radley's role in Bob Ewell's death would be "like shootin' a mockingbird."
The children are harmless as well and exposing them to danger would be akin to doing the same to Radley.
A very interesting symbol, though, that is critical to the understanding of the book is the Mockingbird itself. The Mockingbird has a very deep and powerful meaning in the novel. In general, it represents peacefulness, innocence and kindness. Characters such as the reclusive Boo Radely, the falsely accused Tom Robinson, and even our narrator, Scout Finch all represent parts of the mockingbird. Yet, the mockingbird's influence does not end there. For it can also be applied to relationship between human beings.
JEAN: … When he gave us air rifles, he asked us never to kill a mockingbird.
(MISS MAUDIE ATKINSON has come out on her porch.)
MISS MAUDIE (to JEAN LOUISE). Your father’s right. Mockingbirds just make music. They don’t eat up people’s gardens, don’t nest in corncribs; they don’t do one thing but sing their hearts out. That’s why it’s a sin to kill a mockingbird. (6-7).
Sorry, there is "who", not "what".