All of the civilian characters—Kathleen, Martha, Bowker's father, Sally Gustafson—are important mainly in that they can't possibly understand what the soldiers are going through.
They misunderstand in their own ways, of course. Martha is interested in intellectual things like poetry and literature, and not the realities of war. Kathleen is too young to get it (definitely check out "Symbolism, Imagery, Allegory" for more on her). Norman's father is sunk in the mire of his own war and won't step out of his experience to allow for the difference of his son's. Sally is happy and unaware of her former boyfriend, who's returned from the war. She'd be shocked by his language if she heard it, unable to recognize that it represents fact.