It's too early in the morning for me to be attempting coherent sentences, but here I go.
I think all the characters are somewhat roughly sketched, but I attribute it to Hemingway's inexperience as an author at the time. Fred Henry does stand out as the only truly thought-through character, much the same way as an inexperienced art student's sketches will show a varying amount of detail throughout a drawing, because they only pay close enough attention to the parts with which they have trouble, or in which they hold the most interest. Hemingway strikes me as not yet having refined the craft of character development; he spends the most time on Henry, because he's the most important to him. At this point, he still needs to learn the discipline of creating a whole world and history and personality for each of his characters, even the minor ones. Instead, he's written caricatures; the characters play their specified roles, and feel duely fake. There are exceptions to this, though; I felt that the character of Aymo must have been based on someone Hemingway knew well. There is more behind the character than a person-shaped line-deliverer, so to speak; he has a history that Hemingway does not devulge, but which I felt, even in the sparse lines he delivered.
Another thing Hemingway still struggles with is constistency of character. Henry is the only character we get to know well, as Virgil points out, but what he benefits from in depth, he suffers from in breadth. Ernest writes bad romantic sequences, which is perhaps the reason Catherine is written so poorly as a whole; all of her lines are melodramatic love-talk. When Henry attempts such language, he is equally fakey. Hemingway seems to have drawn his inspiration primarily from the Hollywood films and literary conventions of his day, which tended toward hokey over-exuberance and lots of "darling, darling, darling's." This is perhaps the most glaring inconsistency that I've noticed in the book.
