In an attempt to helpJem, Boo Radley mends and folds Jem's pants on the fence. In chapter 7, Jem tells Scout,

"When I went back for my breeches—they were all in a tangle when I was gettin‘ out of ’em, I couldn’t get ‘em loose. When I went back—. . . When I went back, they were folded across the fence . . . like they were expectin’ me . . . And something else— . . . They’d been sewed up. Not like a lady sewed ‘em, like somethin’ I’d try to do" (Lee, 59).

Harper once again illustrates Boo's kind nature in chapter 8 during Maudie's house fire. While Scout is standing outside in the cold, Boo Radley silently sneaks up behind her and covers Scout's shoulders with his blanket. That morning, Atticus notices the blanket around Scout's shoulders and tells her,

"You were so busy looking at the fire you didn’t know it when he [Boo Radley] put the blanket around you" (Lee, 74).

Toward the end of the novel, Boo Radley saves the children by intervening when Bob Ewell attacks them. After Boo kills Bob Ewell, Sheriff Tate illustrates Boo's vulnerability and defenseless nature by telling Atticus,

"I never heard tell that it’s against the law for a citizen to do his utmost to prevent a crime from being committed, which is exactly what he [Boo] did, but maybe you’ll say it’s my duty to tell the town all about it and not hush it up. Know what’d happen then? All the ladies in Maycomb includin‘ my wife’d be knocking on his door bringing angel food cakes. To my way of thinkin’, 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. It’s a sin and I’m not about to have it on my head. If it was any other man, it’d be different. But not this man, Mr. Finch" (Lee, 280).

When Atticus asks Scout if she understands Sheriff Tate's reasoning, Scout metaphorically applies Atticus's lesson regarding why it is considered a sin to kill a mockingbird by saying,

"Well, it’d be sort of like shootin‘ a mockingbird, wouldn’t it?" (Lee, 280).

Scout's response emphasizes the fact that Boo Radley is a symbolic mockingbird in the novel because he is a benevolent, vulnerable being.

Arthur "Boo" Radley is considered one of the two characters in this story symbolized by the mockingbird. While Tom Robinson is the other, the reason why Boo is considered a "mockingbird" takes us back to the first part of the novel, where Atticus explains that the children are not to shoot at mockingbirds because all they do is sing. They do not molest crops, other animals, or humans. They are completely innocent, harmless birds whose sole purpose is to produce beautiful songs.

Boo is considered a "mockingbird" because he is unjustly treated despite his innocence: That is, the community perceives him as a misfit and as a criminal sort due to stories of his youth. He was alleged to have stabbed his father, Nathan, and another rumor stated that he was kept in the courthouse basement before being "chained" to his bed because of insanity. Because Boo winds up being an innocent man unjustly treated, he can be compared to the mockingbird, just as Tom Robinson is.

Harper Lee's novelTo Kill a Mockingbird depicts a mocking bird as a bird that does nothing, it only sings. It is a living being, basically, that is incapable of hurting anyone because of its size, its lack of malice, and its inability to overtake someone even in self defense.

Similarly, Boo Radley represents a living human being whose nature is not what his accuser wants the jury to believe. He entered the Ewell home with the purpose of helping. He was harassed by the woman and then accused of raping her. This is a direct lie that would make Boo sound like an evil man, which  he is not.

Putting Boo in jail, hurting him, or anything would be like jailing, hurting, or killing a mockingbird:  Something that did not hurt anyone for any reason and whose size in society is so small that he cannot overtake the system even to defend himself. That is the symbolism behind the mockingbird.

If you recall from chapter 10 Scout asks Miss Maudie about something Atticus says--"Shoot all the bluejays you want...but remember it's a sin to kill a mockingbird" (98).  Scout needs clarification because she's never heard Atticus refer to anything as a sin before.  Miss Maudie answers that mockingbirds "don't eat up gardens," "nest in corncribs" and that "they only sing their hearts out for us," which basically means they are harmless creatures who can only bring joy.  Now if you apply what you have come to learn about Arthur Radley, you will find that he is as harmless as a mockingbird and his only true actions have been at the least nice (leaving presents for the children), and, at the most, lifesaving (his actions from Halloween night).

At the end when Atticus mistakenly believes Jem killed Ewell, Sheriff Tate affirms that it was not Jem but Arthur who killed Ewell and Tate continues by telling Atticus that as far as he is concerned Ewell killed himself and justice has won.  When telling this to Atticus Tate specifically uses the word sin to describe exposing Arthur and the truth of the situation.  After Tate leaves, Atticus asks Scout if she can understand but it's Atticus that needs the situation to be explained to him and she has to reaffirm to him that it would be "like killing a mockingbird" (290) reaffirming that it is a sin in the one way that she and Atticus understand: it's simply wrong to bring harm to a creature that only brings joy.

By extension you can see how Scout has grown up and is able to understand that sometimes justice isn't a black and white issue, but more of a grey one.