Why does Hemingway refer to the protagonist as Krebs rather than Harold? What is the significance of his sister calling him "Hare"?
In referring to the protagonist as Krebs, Hemingway also reinforces the isolation that surrounds Harold after his return from war.
Harold feels emotionally detached from the members of his community; the average civilian neither recognizes his war stories as genuine nor understands the significance of war from his viewpoint. To regain his societal relevance, Harold begins to concoct lies about his war experience, "stating as facts certain apocryphal incidents familiar to all soldiers." However, the more Harold lies, the more repulsed he becomes by his stories (and himself).
Harold has lost his easy camaraderie with others, and his emotional angst is making it difficult for him to reintegrate into society. The surname "Krebs" highlights Harold's emotional disequilibrium: he is a soldier who has "lost everything" and who is now unable to love even his own mother. He absently goes through the motions of life and desperately clings to the mundane to provide him a sense of normalcy.
As Harold ponders his life, he notes his sudden aversion to subterfuge and deception. Interestingly, Harold views courtship as a distasteful process replete with "lies." He yearns for a life "without consequences." Harold senses that courtship must be followed by all the responsibilities that marriage entails. Similarly, for a soldier, the allure of heroic ideals is soon replaced by the grim realities of war. These realities can destroy a soldier's sense of humanity, leaving him detached from society.
Thus, Hemingway's use of "Krebs" reinforces our protagonist's sense of helplessness, inadequacy, and sense of alienation.
In the story, the use of the affectionate "Hare" by Harold's sister is also significant. It reinforces the sense of desperation Harold's sister feels and her need to reconnect with her brother. Like her mother, Helen suffers because she has little idea of how to communicate effectively with Harold. Instead, both Helen and her mother fall back on old paradigms, which prove ineffective in their efforts to reengage Harold.
Soldier's Home by Ernest Hemingway is a typical Hemingway story in which a solitary hyper-masculine figure is misunderstood by the women in his life. The sense of masculine viewpoint and a sort of masculine solidarity that existed in Krebs' military life but not in his life after he returned home to a feminized environment is enhanced by the naming conventions used in the story.
In the military it is traditional to refer to men by a last name, preceded by a rank when demanded by formal etiquette. This was also true of schools at the time when this story was written. Within military and school environments, this form of address (unadorned last name) was typically used from superiors to inferiors. Since the first time military recruits or schoolboys heard each others' names, it was in this form, often old friends retained this form of address. It emphasizes the masculine camaraderie that is not understood by Krebs' mother or sister.
The sister uses "Hare" as a diminutive form of Harold to indicate affection for her older brother. This nickname, and the bonding over baseball, illustrate an the affection between the two, reminding Krebs of his lost innocence, although his sister too leads him into a form of inauthenticity in her discussion of being his "girl."