Jem ages from 10 to 13 over the course of To Kill a Mockingbird, a period of great change in any child's life. Jem is no exception to this rule. Interestingly, the changes he undergoes are seen from the point-of-view of a younger sister, which gives a unique perspective on his growth.
Jem represents the idea of bravery in the novel, and the way that his definition changes over the course of the story is important. The shift that occurs probably has as much to do with age as experience, although the experiences provide a better framework for the reader. When the story begins, Jem's idea of bravery is simply touching the side of the Radley house and then only because "In all his life, Jem had never declined a dare." But as the story progresses, Jem learns about bravery from Atticus facing a mad dog, from Mrs. Dubose's fight with addiction, and from Scout's confrontation with the mob at the jail, among others. And along the way, he grows from a boy who drags his sister along as a co-conspirator to a young gentleman who protects his Scout and tries to help her understand the implications of the events around her.
His own sister finds Jem a genuinely likeable boy, if sometimes capable of "maddening superiority." He very much wants to be like his father, and plans to follow him into law. He idolizes Atticus and would rather risk personal injury than disappoint his father. As he grows older, he begins to do what is right even though his decision may not be popular. For instance, when Dill sneaks into Scout's bedroom after running away from home, Jem can only say, "'You oughta let your mother know where you are'" and makes the difficult decision to involve Atticus. Afterward, he's temporarily exiled by his friends, but he maintains the rightness of his decision without apology.
