The two twin sisters were the slaves of the doll Mary-Ellen, they listened to every word she said, Jillian thought it was all in their imagination, and she found it annoying that the doll was all they talked about. Later in the story, Mary-Ellen brings the evil dummy Slappy to life so that Slappy will be her new husband, but Slappy didn't want her, he wanted Jillian. Slappy and Mary-Ellen fight, then he pushes her into a saw, and she pulls him in with her. Now Katie and Amanda are finally free from their slavery.
Katie is described as "the wild one" who is "always wired." Jillian claims Katie never cries (although Mary-Ellen is capable of making her sob in two different parts of the book). Amanda is "a lot cooler, a lot calmer, a lot quieter, and more thoughtful." Jillian claims Amanda will cry if someone "looks at her funny." Both girls will chant when they're excited. Both love pranks and cheer when they win an argument with Jillian, but they also are scared when Jillian significantly loses her temper.
Both love Jillian and wanted to tell her about Mary-Ellen, but Mary-Ellen threatens them into mistreating her.
Katie and Amanda are six year old identical twin girls. Like Jillian, they are described to be tall and thin and have black hair, which Katie wears in a ponytail, and Amanda has hanging loose over her shoulders.