Fred, Holly's brother, and Doc, the man who took her and Fred in as children and who she's married to, are pretty minor characters when it comes to actual action in the story (in fact, we never actually meet Fred and instead just hear about him from Holly). However, we think these two are important nonetheless because they both signify a time in Holly's life before she is the girl we know in New York. It's through Doc that we learn about Holly as a young girl, about the death of her parents, and about the hard life she's forced to lead when she no longer has her mom and dad around. And Holly's affection for the husband who's old enough to be her father shows us a rare glimpse of a sweeter, more sentimental young woman. Doc is one of our few links to Holly's past, and for this reason we think he's worth noting.
And speaking of Holly's past, perhaps no one is more important than Fred. We never meet him, never hear directly from him, and Holly mentions him just a few times. But he is the one person she keeps consistently close to her heart, and the one person we can say without a doubt that she loves and worries about and whose feelings she actually takes into consideration. For most of the story, the life she could have with Fred after he returns from war is Holly's idea of happiness. We see her lose control just once in the story – when she hears of Fred's death. His profound impact on her is what makes this "minor" character not so minor after all. Like Doc, Fred is a vehicle through which we learn about our protagonist from someone other than the narrator, and like Doc, his presence in the story might make us like Holly just a little more. Consider this statement from Holly about the brother she loves so much:
"Everybody thought it was dotty, the way he gorged himself on peanut butter; he didn't care about anything in this world except horses and peanut butter. Be he wasn't dotty, just sweet and vague and terribly slow; he'd been in the eighth grade three years when I ran away. Poor Fred. I wonder if the Army's generous with their peanut butter." (3.22)
Holly has very little patience for most people's quirks, but she seems to love Fred all the more because of his. She remembers her sweet, not-too-bright brother and worries that he's not getting what he needs in the Army, and this description of her brother tells us a lot about her.