Boo Radley is typically considered one of the "mockingbird" characters in this story.  Remember Atticus' lesson to Jem, the day he gets his first gun:

Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird. (Chapter 10)

Mockingbirds do not do anything to hurt anyone.  They simply make music for others to enjoy.  Boo Radley leaves gifts for the children in the tree.  There is evidence by the end of the novel that he is completely aware of their morbid curiosity in him, and yet he never retaliates.  In fact, he slowly reveals more of himself.  By the end of the book, he is the one who ultimately saves Jem and Scout by killing Bob Ewell in the dark.

Despite the fact that Boo Radley is the victim of an unjust punishment by his father, and further unjust treatment (socially speaking), he sacrificially reveals and allows himself to reach out to these two children in town.  Two children who are very much like him, but unaware of it.  Because of the Tom Robinson case, Jem and Scout are also targeted for scorn and ridicule, but in many ways, they too are mockingbird characters.  It is as if Boo recognizes this, and makes a connection with them because of it. 

Readers can learn many lessons from Boo.  First, we can learn not to let our circumstances (however unfair they may be) define us.  He rises above the status of "victim" and in the end becomes a hero.  We can also learn that it is okay not to take credit or "show off" in the good things we do.  In this way, Boo teaches readers a true sense of humility.  Finally, Boo is a lesson in non-prejudicial acceptance.  Despite the fact that he is unfairly judged, he does not pass judgement on others.  Certainly, he does not pass judgement on the family of the man defending the black man in court.