The Cuckoo Sister features several important characters: Kate Seton, her parents Anthony and Margaret Seton, housekeeper Mrs. Trapp, Kate's godmother Elizabeth Wait, and Rosie Martin—who may be Emma Seton. Louise Martin—who raised Rosie and whom Rosie believes is her mother—is also important, although she is known to the reader only through her actions, her ungrammatical letters, and the dialogue of others. Brief roles are also played by several interested paternal relatives of Kate, several of her snobbish friends, Rosie's colorful and concerned Hockley acquaintances, and Harry Jenkins, Louise Martin's boyfriend-turned-husband who knows the truth about Rosie's identity.

Kate and Rosie are the most important characters. Kate is striking as the narrator and the one who changes most in the course of the novel. At the outset eleven-year-old Kate is a pampered busybody—untrustworthy, rude, jealous, and crybabyish...