Most people are unknowable. Hemingway uses a technique known as the Iceberg, which is to say that, on the surface, there may not be much there for your to grasp at, but digging deeper into the subtle symbols, phrases, and particular arrangement of words, you begin to understand the meaning of the story, just what he was trying to say with these characters. By trimming away anything that would too obviously give the character away, the reader makes certain judgements based on their own perception of the characters, rather than what Hemingway wants you to feel about these characters. It's the same thing that we do with people when we judge them from afar. Hemingway has a very interesting way of bringing up your own, ephemeral experiences and manipulating them to convey the true story outside of the words that he's written. He plays with the Lovecraftian idea that the imagination is the strongest tool imaginable to a writer. Hemingway honed his characters not by crafting very specific characters, but crafting characters who were real in the sense that they were not of one face.
Hemingway's notorious concept of ambiguity in his texts constructs an accurate portrayal of relationships and contributes to the development of the characters' complexity. I am unable to articulate how he does this through his narrative style but it appears that doubt and ambiguity provides an insight to the human condition.