Although she actually appears only very briefly in the book, Edmund's "Sis" is a formidable character. In the prologue, it is she who urges Edmund to take action and fetch them some food when their Aunty does not return for two days. Aunty has given the children firm instructions to stay inside the apartment, and Edmund is inclined to follow her directions to the letter. Sis, however, is not content to just sit indefinitely and starve. She tries to convince Edmund that the two of them should take the money they have and go to get some food, and, failing to get him to consent to this, she suggests that he, at least, should go out, while she stays behind. When Edmund finally agrees to go, Sis promises to take responsibility for the decision should their Aunty return.
Sis is, of course, kidnapped, and Edmund spends the course of the narrative searching for her. Even in her absence, however, Sis shows her intelligent and "plucky" character, leaving behind clues to her whereabouts in the form of buttons, like Hansel and Gretel. Sis's resourcefulness and spirit enable Edmund and the mercurial Mr. Dupin to solve the mystery of her disappearance and rescue her.
Sis reappears at the end of the book, and again demonstrates her bravery and feistiness. She is seen struggling with her captives, and courageously leaps from a boat into the water, managing to throw the villains into confusion in order to elude them and escape to safety.