In Harper Lee's To Kill a Mockingbird, the theme of appearance vs. reality is a prevalent one, and Boo is a perfect example of this theme—as is Tom Robinson, at least as far as the townspeople are concerned.
When Atticus decides to defend Tom Robinson for allegedly raping Mayella Ewell, the townspeople are angry. They anticipated that Tom would have a lawyer, but not one that would defend a man that most of them believe raped a white woman. Because Tom is black, he is guilty before he steps into the courtroom—as seen by the lynch mob that shows up at the jail while Atticus sits outside with a light, chair and newspaper. The mob has made up its mind that Tom is guilty, and they do not need a court trial to carry out the sentence they believe he should serve: to be hanged until dead.
"He in there, Mr. Finch?" a man said.
"He is," we heard Atticus answer, "and he's asleep. Don't wake him up."...
"You know what we want," another man said. "Get aside from the door, Mr. Finch."
These men are sure Tom is at fault based on his appearance—the color of his skin. We discover during Tom's testimony in court that Mayella actually kissed Tom (ostensibly because she was so lonely). Her injuries were not sustained at Tom's hand, but at her father's when he saw her embrace Tom through a house window.
Boo is another example of appearance vs. reality. Few people see Boo, so many rumors fly around, often shared by Miss Stephanie, the town scold (or "gossip"). When he saves the children, he hardly looks like someone who could have stabbed Bob Ewell to defend Jem and Scout from Ewell's attempt of murder. However, Heck Tate figures it out rather quickly. Whereas Atticus is sure that Jem killed Ewell and thinks Heck is trying to save Jem for Atticus' sake (another example of appearance vs. reality...for Jem did not do it), Heck understands that Arthur "Boo" Radley has risked his life to save the children. And in doing so, he has exposed himself to society: he can be protected, or he can be introduced as a hero: which Heck sees as unacceptable.
To my way of thinking, Mr. Finch, taking the one man who's done you and this town a great service an' draggin' him with his shy ways into the limelight—to me that's a sin...If it was any other man, it'd be different. But not this man, Mr. Finch.
Boo is not someone accustomed to being out in public:
They were white hands, sickly white hands that had never seen the sun...His cheeks were thin to hollowness...there were shallow, almost delicate indentations at his temples, and his gray eyes were so colorless I though he was blind.
Boo is nervous, with sweaty palms that streak the walls where he had rested them. He is shaken by spasms. His face is tense and it only relaxes looking at Scout's face. His smile is timid. When he speaks he sounds like a child.
Looking at Boo, one could hardly imagine that he was strong enough emotionally or physically to survive a fight with the drunk Bob Ewell, let alone kill him, but Boo does so—driven by his love for Jem and Scout, who he has been watching all summer. His concern for them overrides his desire to remain hidden: as he did at the fire after the snowstorm, and as he was when he mended Jem's pants and left them on the fence.
Boo is an unlikely hero in the story: the perfect example of how one's appearance is often misleading. We have heard, "Don't judge a book by its cover" for this reason. The appearance of Boo is unlike the reality of the man.