In chapter 15, Jem demonstrates his developing maturity. Scout mentions that her brother no longer engages with Dill and her in some of their activities. Additionally, Jem also seems to be more observant of the adult world and the interactions of his father with his sister, Alexandra.
After Scout overhears her father arguing with Aunt Alexandra, she seeks Jem in his room where he seems to be very pensive. Since she has heard her father arguing with Aunt Alexandra, she asks Jem about their confrontations.
“Have they been at it?” I asked.
“Sort of. She won’t let him alone about Tom Robinson. She almost said Atticus was disgracin‘ the family. Scout. . . I’m scared.”
“Scared about Atticus. Somebody might hurt him.”
Jem's anxiety for his father demonstrates his maturity, a maturity that later proves to be valuable. That evening, Atticus departs, saying that he will be gone for a while and everyone will probably be in bed when he returns. Later, Scout hears Jem stirring in his room. She asks Jem what he is doing, and when he tells her that he is going to look for Atticus, Scout insists upon accompanying him. Before they go downtown, they wake up Dill, and he eagerly goes along. The three children seek Atticus at his office in the bank building, but he is not there. Instead, he sits in a chair, propped against the jailhouse door. The cord and light that Atticus took with him are overhead as he reads his newspaper. Jem tells Scout and Dill that they can leave; he has just wanted to know where Atticus was. However, at that moment, the children hear cars pulling in near them. They run around and hide where they are out of sight. When Scout overhears her father talking with the men, he uses a phrase that he often says when playing checkers. Scout does not realize the danger, and she races to see her father. Jem hurries after her; Atticus tells Jem to go home and take Scout with him.
Jem shook his head. As Atticus’s fists went to his hips, so did Jem’s, and as they faced each other I could see little resemblance between them: Jem’s soft brown hair and eyes, his oval face and snug-fitting ears were our mother’s, contrasting oddly with Atticus’s graying black hair and square-cut features, but they were somehow alike. Mutual defiance made them alike.
Significantly, Jem defies his father for the first time. Jem does so because he does not want Atticus to be alone as he faces the mob. Certainly, Scout cannot help but notice the maturation of her brother.