Over the course of To Kill Mockingbird Jem and Scout develop along the novel.
Jem is the older one of the siblings - he is 10 at the beginning of the story and at the end he is 13 - but at the same time, he has childish behaviors, such as when he kills Mrs. Dubose’s flowers because she called his father names. But along the story, he develops responsibility and maturity since he recognizes that the situation his father is handling is serious. He respects his father and develops a sense of justice, so he knows that he must do right even if his decision may not be a popular one.
Scout also matures along the story, at the start of the novel she is a tomboy girl who is a little aggressive. She does not understand, at the time the implications of the things that are happening around her. But with time she learns some lessons and becomes more self-controlled. This is clearly seen when she does not fight Cecil because she knows this behavior would let her father down.
At the end of the novel she learns to put herself into the place of others, she learns with her father’s case to see from the other person’s point of view and with this she puts herself in Boo Radley’s shoes.