The most central example of the mockingbird is Tom Robinson. He, like the mockingbird, has done nothing but be nice to people and live his life in peace. However, he is the target of suffering and violence. A quote that demonstrates this is from Chapter 23:
The witnesses for the state.have presented themselves to you gentlemen, to this court, confident that you gentlemen would go along with them on the assumption-the evil assumption-that all Negroes lie, that all Negroes are basically immoral beings, that all Negro men are not to be trusted around our women, an assumption one associates with minds of their caliber. Which, gentlemen, we know is in itself a lie as black as Tom Robinson's skin, a lie I do not have to point out to you." Atticus, page 217.
Here's another one:
".As you grow older, you'll see white men cheat black men every day of your life, but let me tell you something and don't you forget it-whenever a white man does that to a black man, no matter who he is, how rich he is, or how fine a family he comes from, that white man is trash."-Atticus (233)
The other character that is a mockingbird is Boo Radley. Boo has suffered at the hands of his father and his town, although he's never done wrong. He loves and helps the children, even saving their lives. Scout sums it up best, when she says that if Boo were to be put on trial:
"it'd be sort of like shootin' a mockingbird" (291)