Who is The Kleins from Z for Zachariah and what is their importance?

The Kleins were an elderly couple who owned a store not far from the Burden farm. The Kleins lived in a small apartment above the store. Before the nuclear weapons came, the Kleins sold a great many of their products to the nearby Amish families. For this reason, the store the Kleins owned before the war is well stocked with many supplies, including food, seeds, and fertilizer.