Final answer:
Walter, Coleman's brother, fought in World War I.
Step-by-step explanation:
Walter, the brother of Coleman, fought in World War I. Although there is no specific mention of Walter's role or unit, the passage mentions the experiences of other soldiers during the war which provides context. For example, Grace's brother Basil was a part of the No. 1 Canadian Tunneling Company and her brother Ramsey was a lieutenant in the 38th Battalion. These examples indicate that Walter likely fought in World War I as well.