What girl does Cassie get back at in the story?

Lillian Jean is Jeremy Simm's older sister. She is mean, bossy, and believes herself to be better than Cassie because she's white. Cassie gets her back by pretending to befriend her and coaxing her into the woods. When Cassie throws her books in the dirt, Lillian Jean slaps her. In Cassie's eyes, the slap is good reason for an all out fight.