Cassie embarks on her "friendship" with Lillian Jean well aware that it can never be real. Lillian Jean does not suspect that Cassie's friendship is an act, and she continues to be surprised even after Cassie beats her up.... she has no clue that Cassie doesn't actually want her friendship. Cassie, on the other hand, does not share Lillian Jean's naivety and takes care to not leave any marks on Lillian Jean's face, which is both pragmatic (there is no evidence against her) and metaphorical, because it shows that Cassie understands the importance of appearances in a way that Lillian Jean does not.