This whole novel is about Mockingbirds who do nothing but sing so we can enjoy their song and never do any harm. Certainly I agree with the ones listed, but I would say Atticus himself is one of the most obvious examples of a mockingbird. He suffers the prejudice and insults of the bigots in town and nearly gets caught between a lynch mob and Tom Robinson, but all he's trying to do is to make the world better by bringing truth and justice into it.
And I'm definitely one that sees the Mayella as a mockingbird all the way up to the point where she goes along with her father's rape accusation against Tom. She's most likely sexually abused, and yet she tries to care for her younger brothers and sisters and raise those dang flowers of hers. But faced with the town's hatred and racism and given a choice between admitting that she initiated sex with a black man or making a false accusation, she gave up her mockingbird status by striking out with the viciousness of a vulture.
Jem and Scout Finch may also fit the symbol of a mockingbird because they are innocent children who are attacked by Bob Ewell. Mr. Ewell is trying to get back at Atticus for defending Tom Robinson against Mayella's rape allegations. In a drunken stupor, Bob Ewell attempts to kill Jem and Scout after the Halloween pagent. If it hadn't been for Boo Radley, the children would have suffered although they had done nothing wrong. As it was, Jem suffered a broken arm, and Scout's childhood innocence was lost as she witnessed the cruelty of prejudice in Maycomb.
If you interpret a "mockingbird" as a character who is harmless--or even helps someone--yet is victimized, you can identify these characters. Boo Radley is regarded as weird and shunned by the town, but he tried to befriend the Finch children and ends up saving their lives. Tom Robinson helps Mayella move furniture because he feels sorry for her, but he is falsely accused of rape. Mayella Ewell may seem to be an unlikely candidate because she attempts to seduce Tom and is responsible for the false charge, but she has an abusive father and a cruel home life. She, too, is a victim who is looking for some affection in her life; her father pushes her to accuse Tom.
I was thinking Calpurnia, because she does no harm, and she supports the Finch's. And when Aunt Alexandra comes into the story, she always wants Atticus to fire her. Mayella is also an option as well. Does anyone think that Calpurnia would be a symbol of a mockingbird?