Who is Zoomer from What It Is Like to Go to War and what is their importance?

Zoomer is a young Marine serving under Marlantes' command. He took an extensive would to the chest during battle and appears in the book's opening scene. When we meet him, Zoomer is gasping for life while waiting to be evacuated. Marlantes is nearby, whispering stories in order to keep him awake until the evacuation takes place.