Generally, we as readers come to understand a character in a story better and better either as that character better understands themselves (if they are the protagonist) or as others characters grow in understanding of the person. In this case, since Cassie is the protagonist, we learn more and more about her as she learns more and more about the world and decides how she will respond to her new and widening experiences.
At the beginning of the novel, Cassie has little experience with racial prejudice and oppression. She is still quite young and has largely been sheltered up to this point. That is about to change, though, and Cassie has to decide how to react both to the events around her and to her own emotions about those events. As she does, she grows.
We readers watch as Cassie grows, and we get to know her better and better. We see how upset she is when she discovers the “new” textbooks at school are far from new. We watch as Cassie struggles to grasp the reasons why her parents and neighbors are refusing to shop at the Wallace store. Cassie also learns the horrors of violence as she hears about men burned alive simply because of their race. We experience firsthand with Cassie the frustration of being treated as dirt by Lillian Jean and the store clerk. Cassie struggles with the injustice of having to apologize to Lillian Jean for no reason.
We also learn more about Cassie as we see her struggle to figure out what is right. She doesn't want her brother Stacey to take the blame for something T. J. has done, but she leaves the decision to her brother and remains silent. Cassie carries out her own plot against Lillian Jean, and we have to admire her cleverness but perhaps not her methods. We also feel Cassie's mix of confusion and anger as she watches T. J. get into more and more trouble, and we understand her fear the night when the men come to lynch T .J. and perhaps her own father, too. She doesn't comprehend the meaning or purpose of the fire at first, but by the end of the novel, she understands and admires her father for his courage.
Indeed, by the end of the story, Cassie understands much more about herself and her life, and we readers have gotten to know her well.