What is Jem's character motivation as he relates to Boo Radley?
At first, Jem buys into the rumors about Boo being a monster. He is as infatuated with Boo as Scout and Dill are. In Chapter 1, Scout gives Jem's description of Boo:
Jem gave a reasonable description of Boo: 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.
In Chapter 5, Atticus reprimands the three children for making fun of Boo. Jem's takes exception to this, still thinking that it makes sense to mock someone who chooses to hide in a house for years. Jem's perception of Boo starts to change at the end of Chapter 6 and the beginning of Chapter 7. When he goes back to get his torn pants, he finds that they have been sewn. Harper Lee doesn't make it clear that Jem knows Boo fixes the pants, but given the events that follow, Jem would probably have come to that conclusion. At the end of this chapter, Boo's father puts cement in the holes in the tree, thus ending the parade of gifts. Jem seems to realize that they had made a connection with Boo through this game. He is sad when Mr. Radley puts a stop to it. As Jem matures, he is more able to consider the perspectives of other people. It's safe to assume that by Chapter 23, he no longer thinks of Boo as a monster. In fact, when discussing how people of different classes and races don't get along, he comes to the conclusion that Boo is perhaps smart for staying indoors:
Scout, I think I’m beginning to understand something. I think I’m beginning to understand why Boo Radley’s stayed shut up in the house all this time . . . it’s because he wants to stay inside.
Through his interactions with Boo and his interest in Tom Robinson's trial, Jem learns empathy. He also learns about prejudice with how the jury misjudges Tom and how people have unfairly judged Boo Radley.