Who is Sarah Lumb from The Eye in the Door and what is their importance?

Sarah is Billy Prior's steady girlfriend. Sarah lives in northern England and works in a munitions factory, like many young women of the time. Sarah is portrayed as free-spirited and lively as well as very committed to Billy. Sarah is aware of Billy's fugue states and has even experienced one of Billy's fugues firsthand. However, she stays with Billy because she loves him. Sarah does know that Billy is bisexual.