Who is Thomas Michael McCoy from Line of Fire and what is their importance?

Thomas Michael McCoy is a sergeant in the United States Marine Corps. He is twenty-one years old and usually in trouble. He assaults one or more commissioned officers and is court martialed, sentenced to dishonorable discharge and to ten years of hard labor. The sentence is set aside because of irregularities in the trial and he is sent to Camp Elliott where he answers to Gunnery Sergeant Ernest Zimmerman, who is a friend of his brother Ken, who is three years older. After a fight with Zimmerman which puts him in the hospital for several days, his attitude changes and Zimmerman recommends him for a promotion. He performs well at the Makin raid but then beats up a sergeant afterwards. While in prison in Honolulu, he and Zimmerman are transferred to the MAG-21 on Guadalcanal.