A Washer is a specially trained telepath who can erase minds and memories. Sophie's human family—William, Emma, and Amy Foster—had their memories of Sophie wiped from their minds by a washer (Book 1: Keeper of the Lost Cities). In Book 4: Neverseen, Keefe found memories that were washed from his mind by Lady Gisela's order. In some of these memories, Lady Gisela calls for, or asks some one else to call for, a Washer. This Washer is presumed to be Gethen because he mentions he was recruited for his washing abilities. One Washer is mentioned in Book 6: Nightfall when Sophie's human parents' memories had to be erased for their sanity.
He mentions that he was recruited to the Neverseen for his superior washing abilities.