There is more than one hero in To Kill a Mockingbird, but Boo Radley is a special kind of hero.
Boo Radley is interesting to discuss as a hero because in the beginning of the book he is a villain, in the legendary sense.
The children are fascinated with Boo Radley. He never comes out, and he has an interesting story of madness and violence, but also with an undertone of sadness.
The Radley Place was inhabited by an unknown entity the mere description of whom was enough to make us behave for days on end… (ch 1)
The children have detailed fantasies about Boo Radley. They imagine him as violent, eating animals raw and with jagged scars. They also act out the details of his life, first as part of the fantasy, and then with truthful elements stitched together from neighborhood gossip.
The children desperately want to make Boo come out. They even try to convince him that they are his friends. Slowly, their overtures get results. It is soon clear that Boo is watching them—and watching over them—from the safety of his house.
Boo begins by leaving presents for the kids, and intervening in their safety in small ways. He puts a blanket over Scout’s shoulders as she watches the fire, and he saves Jem from punishment by mending and returning his pants. The children begin to see him as a friend, and are saddened when he no longer leaves them gifts.
Boo also sees when Bob Ewell follows the children and tries to attack them with a knife. He intervenes, saving the children and fatally stabbing their attacker. Heck Tate and Atticus agree to keep this a secret, because Boo’s heroism would bring him fame he would not be able to handle.
Atticus is fully aware of what a hero Boo is. Before Boo returns home, he tells him so.
"Thank you for my children, Arthur," he said. (ch 31)
Not all heroes are outgoing. Some stay quietly in the shadows and behind the scenes.