James did not like the characters he created in his novel; he felt that they were under-developed and needed to be fleshed out a little in order to make greater thematic sense. Critics, however, disagreed with him, recognizing immediately the intentions of the novel and the characters in it. James wanted readers to know that different people were motivated by different things, and that one person's misfortune was another's opportunity if viewed from a different lens.
The main lens through which we see Milly in the novel belongs to Kate, whose duplicitous plan to inherit Milly's fortune forms the outline of the novel's plot. At first she appears genuine in her offers of friendship, but Kate's interests wander away from helping Milly and toward helping herself, especially after she visits the doctor with Millie and realizes that Millie is incurably ill. This realization is partly the fault of the physician because his answers to Milly's questions are so crushingly non-committal that Kate can't help but believe Milly's illness to be imminently fatal.
Kate's plan is simple: take advantage of Milly's situation, and of Milly herself, while simultaneously acquiring a nest egg for herself and her husband-to-be. It is difficult to tell whether or not she loves Merton as much as he loves her, but we can tell that she loves the idea of him rich more than she loves the actual Merton to whom she is betrothed.
James shows humans with frailties. Even Milly is not immune from this, although her frailties are seen as tragic and beautiful--such as a tendency to fall in love too quickly, or a tendency to trust or to believe that everyone is as nice as she is. Kate is the most deceitful and manipulative character, and the other characters fall somewhere in between. Tragedy for Milly is also opportunity for Kate and Merton, and Kate decides to grab hold of this opportunity with both hands.
It is not clear at the end of the novel whether or not Kate agrees to turn down Millie's fortune in order to marry Merton. She admits that their relationship will be forever different, but it is left to the individual reader to decide whether or not if the pair will ultimately stay together.
