The person that, undoubtedly, changes the most is Jem. Even before the trial, there are some inklings of maturity. However, the trial definitely changes Jem and is his outlook concerning life.
When the verdict of guilty is placed on Tom Robinson, Jem does not take it well. He knows at the core of his heart that Tom is innocent, which means he also knows that Maycomb is racist. The text records how hard Jem took this news.
His face was streaked with angry tears as we make our way through the cheerful crowd. “It aint’ right," he muttered all the way to the corner of the square where we found Atticus waiting... “It ain’t right, Atticus,” said Jem. No son, it’s not right.” We walked home.
Later Jem's maturity is made even more apparent. When Bob Ewell attacked the children. Jem protected Scout with little worry for himself. He really embodied sacrifice and courage, the very qualities that Atticus possessed.
Another character that changes significantly is Scout. At the end of the story, in some ways, she is wiser than Atticus. She realizes that Boo Radley was also a mockingbird, a point that Atticus had a difficult time seeing. Scout probably came to this conclusion by observing how society worked in Maycomb, especially at the trial of Tom Robinson.