Who is Narrator/Staff Sargeant X from Nine Stories and what is their importance?

Staff Sergeant X is described as a thoughtful, educated, literate man, whose military career begins in a quiet section of England. He arrives in Europe on D-Day, and he is present throughout the fighting, clear through the end of the war on V-E day. X emerges from the fighting a changed man and is hospitalized because because of a mental breakdown. His recovery begins after reflecting on a conversation he had before the fighting with a charming young English girl named Esme.