Who is Gunnery Sergeant Elmo M. Haney from With the Old Breed, at Peleliu and Okinawa and what is their importance?

Gunnery Sergeant Elmo M. Haney is a peculiar individual who is the gunnery sergeant for K/3/5, Sledge's combat company. Haney is a World War I veteran, as well as a veteran of Guadalcanal and Cape Gloucester, and over fifty years old at the time period considered by the text. He is of slight stature and weighs in around 135 pounds, though he is heavily muscled. He is a loner and often talks to himself. He uses a stiff bristle brush to scrub his genitals in the shower, cleans his weapons three times a day, and spends an hour daily muttering and chuckling while performing bayonet drills; he is obsessed with bayoneting the Japanese.