How does Jem change throughout To Kill a Mockingbird?

Jem matures into a young man who cares about other people and no longer fears neighborhood monsters. 

At the beginning of the book, Jem is still very much a child.  He enjoys playing make-believe games with his Scout and Dill. He fears Boo Radley. He is a sensitive and intelligent boy, but, at age ten, he is still a boy.

Like Scout, Jem has to grow up during the Tom Robinson trial. He struggles to find himself and his place in the world. For example, when Jem runs away from the Radley house and loses his pants, he does not want to tell Atticus where they are. He is not afraid of being punished, worrying instead that Atticus will think badly of him. 

Jem develops empathy for Boo Radley before Scout does.  He realizes Boo put the blanket on Scout’s shoulders during the fire and mended his pants for him. Jem pleads with Atticus not to tell anyone, as he does not want Boo to get in trouble, as evident in this quote from Chpater 8:

Mr. Nathan put cement in that tree, Atticus, an‘ he did it to stop us findin’ things—he’s crazy, I reckon, like they say, but Atticus, I swear to God he ain’t ever harmed us, he ain’t ever hurt us, he coulda cut my throat from ear to ear that night but he tried to mend my pants instead… he ain’t ever hurt us, Atticus.

During the trial, Jem feels very involved in the process. He is old enough to follow the proceedings, but still naïve enough to think Tom Robinson will be acquitted. Jem follows the evidence, not the way people actually think. He feels cheated when Robinson is convicted. This is the first time he really understands racism. 

Scout and Jem begin to grow apart as Jem enters junior high. He eats a lot, is moody, and shows her new places on his body where hair is growing. He also tries to make Scout mind him, and Atticus says she has to when he can make her. 

When Jem is attacked by Bob Ewell and breaks his arm at the end of the book, all Jem cares about is that he can still play football. Jem wanted Atticus to stand up to Bob Ewell, and did not understand why he would not carry a gun. That was just Atticus’s way. At the end of the novel, Jem is twelve-years-old, almost a teenager.