The main characters of Many Waters are twin brothers, Sandy and Dennys Murry, who, unlike their unusual scientist parents and their time-traveling siblings, Meg and Charles Wallace, see themselves as fairly normal. In A Wrinkle in Time, the twins are the only members of the family who fit in at school and who can get along well in social situations. In Many Waters, the two boys also see themselves as the practical members of the family, the ones who can solve problems, the ones who can do anything. The boys, now fifteen, still enjoy playing sports, like hockey, but are beginning to think about their futures. Sandy wants to be a lawyer and Dennys, a doctor. (That they will both be successful in these endeavors has already been established in A Swiftly Tilting Planet in which they are much older.)

While the twins are...