What is Boo Radley--archetype, stereotype, cliche, symbol, catalyst, foil, and social example of?To Kill A Mockingbird

In Harper Lee's "To Kill a Mockingbird," Boo Radley becomes for the children a prototype of the bizarre recluse who is to be feared.  For the town of Maycomb, Alabama, he is simply the stereotypical odd recluse with which many Southern towns were acquainted since there were few mental institutions where such people could be "sent off."

The mockingbird mentioned in the title becomes symbolic of Tom Robinson and Boo Radley both since, as Miss Maudie instructs the children, mockingbirds never bother anyone;

...they just sing their hearts out for us.  That's why it's a sin to kill a mockingbird.

As the mockingbird, Boo tries to bring enjoyment to the children while displaying his love for them by repairing Jem's pants when he rips them on the Bradley fence, and by leaving little gifts for Jem and Scout. Certainly, with his tenderness and concern for the children, he acts as a foil to Bob Ewell, whose vindictiveness against Atticus Finch is turned upon the innocent Scout and Jem as they return from the halloween pageant. Of course, the significance of the title becomes evident with Boo's heroic gesture.  For, as the sheriff tells Atticus, it would be a shame/"sin" to incarcerate Boo/"a mockingbird" for having killed Ewell.

Boo Radley's presence in the novel acts as a catylst for the motifs of superstition and appearance, motifs which play into the maturation of the narrator, Scout.  As she matures, Scout realizes that the "haints" and the bizarre Boo Radley are not what she has believed.  Instead, Scout sees that Boo is a delicate person who must hide from the harsh world; yet, he is brave and protective:

Atticus was right.  One time he said you never really know a man until you stand in his shoes and walk around in thhem.  Just standing on the Radley porch was enough.

His courage in coming out to defend Scout and Jem exemplifies how misunderstood Boo has been.  So often in a small society such as Maycomb do people prejudge a person such as Boo, believing him malevolent because they fail to understand them.  In reality, Boo is a true hero.