Boo Radley keeps to himself. He is isolated from society. This makes him vulnerable. It is easy for others to make judgments about Boo because he is seldom seen outside. In chapter one, Jem describes him as some type of monster:
"Boo was about six-and-a-half feet tall, judging from his tracks; he dined on raw squirrels and any cats he could catch, that’s why his hands were bloodstained—if you ate an animal raw, you could never wash the blood off. There was a long jagged scar that ran across his face; what teeth he had were yellow and rotten; his eyes popped, and he drooled most of the time."
Due to Boo's isolation, he is vulnerable to such descriptions. Because no one sees him very often, they speculate on what they think he looks like.
In chapter 6, Scout gives her interpretation of Boo Radley;
"Every night-sound I heard from my cot on the back porch was magnified threefold; every scratch of feet on gravel was Boo Radley seeking revenge, every passing Ne_ gro laughing in the night was Boo Radley loose and after us; insects splashing against the screen were Boo Radley’s insane fingers picking the wire to pieces; the chinaberry trees were malignant, hovering, alive. I lingered between sleep and wakefulness..."
Again, Boo is subject to such descriptions because he keeps himself isolated from others.
When Boo begins making contact with Jem and Scout, he does so in an exclusive manner. He does not directly make contact with them.
Nevertheless, by leaving small gifts in the knothole of the tree, Jem and Scout begin to see another side of Boo. Before, they have only heard rumors about Boo. This has made Boo Radly vulnerable to what society thinks of him. Now, through the trinkets Boo leaves in the tree, Jem and Scout are getting to know Boo in a different light. They begin seeing him as a caring human being and not some monster.
Because Boo is quiet and rarely seen outside, he is vulnerable to the rumors of the community. The community has made accusations about what type of person Boo is. He is judged as some type of dangerous monster. Boo is vulnerable in that people do not really know him.
Jem and Scout are beginning to know Boo. He is becoming a caring individual. He sews Jem's pants. He places a blanket on Scout the night Miss Maudie's house burns down.
"We had almost seen him [Boo] a couple of times, a good enough score for anybody. But I still looked for him each time I went by. Maybe someday we would see him. I imagined how it would be: when it happened, he’d just be sitting in the swing when I came along. “Hidy do, Mr. Arthur,” I would say, as if I had said it every afternoon of my life. 'Evening, Jean Louise,' he would say, as if he had said it every afternoon of my life, 'right pretty spell we’re having, isn’t it?' 'Yes sir, right pretty,' I would say, and go on.
Boo is becoming more and more visible to the children. Most importantly, Boo saves Jem and Scout's lives on the night Bob Ewell attacks them. Boo puts himself in danger as he stops Bob Ewell from attacking Jem and Scout. Boo is interpreted in a different light. He is a caring human being who risked his life for the children. Boo is losing his vulnerability.