Out of all of the characters in To Kill a Mockingbird who experience some sort of character development, Jem changes the most. Scout certainly changes as well, but because she is so young, the events of Tom Robinson's trial do not affect her in quite the same way as they do her older brother. At nine years old, Jem is more mature than Scout and better able to grasp the consequences and implications of the jury's verdict against Tom, which is, in large part, why it affects him so deeply.
While Jem, Scout, and Dill often played silly games—like the Boo Radley game—together at the beginning of the book, Jem was already moving away from childhood before true his loss of innocence at the trial. Scout complains that Jem is growing disinterested in their childish games. She observes that Jem is taking himself very seriously and increasingly seeks to imitate his father, Atticus, whom he sees as noble, wise, and just. As he grows older, Jem begins to notice how class and race divide people in ways that Scout cannot fully understand, such as when he tells Scout that there are four kinds of people in the world:
There’s the ordinary kind like us and the neighbors, there’s the kind like the Cunninghams out in the woods, the kind like the Ewells down at the dump, and the Negroes.
When Scout naively insists that "there’s just one kind of folks. Folks," Jem responds that things are not always so simple:
“That’s what I thought, too,” he said at last, “when I was your age. If there’s just one kind of folks, why can’t they get along with each other?
Though Jem is relatively mature, he is still a child for most of the book—that is, until the trial is over. At Tom Robinson's trial, Jem's faith in justice and his faith in his father's abilities as a lawyer prevent him from believing that racism could win out over reason and evidence; he is smiling and excited after Atticus's defense, certain they have won:
"...don’t fret, we’ve won it,” he said wisely. “Don’t see how any jury could convict on what we heard—”
When, despite his obvious innocence, Tom is deemed guilty by a jury of white men, Jem is crushed. The realization that the adult world is not always good or reasonable triggers a loss of innocence for him, and for some time after the trial, Jem remains jaded and cynical. While Atticus assures Scout that Jem will eventually recover from the incident, Jem's view of the world will undoubtedly never be the same.