Who is Ann Burden from Z for Zachariah and what is their importance?

Ann Burden is a teenage girl who is the sole survivor a war that ended in nuclear holocaust. Ann's valley was protected from the nuclear fallout because of some meteorological phenomenon. Ann's family and their neighbors, the Kleins, also survived the initial attack, but later disappeared while exploring the area outside of their valley. This left Ann alone, unsure if anyone else in the country survived.