The relationship between Hartley and Mitsy is a complex one. This may be due to the many faces of both parties. In the novel, Hartley starts off carefree, but progressively becomes dismal (through most of the book), loving, and hate-filled as the setting of the book changed (possibly the hate should be an exception, as these two occasions were "spur of the moment"). He even showed an act of cowardice when, instead of rescuing Jamie instantly, he lingered, wondering if his hate for Jamie should be good enough cause to let him die. Mitsy resembled a two-faced being during many parts of the book, an example being when Hartley described her in the internment camp, "I began to recognise two Mitsy's: one the agitator and one the lover." (page 120). She shows many emotions: hate, love, grievance, mischief and understanding.
She could be classified as a different type of person from just this mix of emotions. The fact that their feelings never really match throughout this whole book may be a cause for their weak relationship. Another possibility could be in the turning points in the book. The first is where Mitsy misinterprets Hartley's head signal, and assumes a hurt state.
The consequence was that she and Hartley were barely in any contact with each other. The turning point here illustrates the fact that they cannot read each other. This is definitely not a quality of any relationship. The other real turning point is where the Sennosuke's are brought into the Penrose's home. Hartley and Mitsy become quickly intimate, but it breaks off as the wireless brings in news of the war, particularly about the progress of the Japanese.
When the letter is received, Hartley, out of sheer despair, verbally attacks Mitsy, saying "You bitch!" . Although he didn't fully mean it, the damage was done, and Mitsy started to see Jamie socially. From what we see with the last two "turning points", it seems that Hartley is doing the real damage to their relationship. This may also contribute to the fact that their relationship is quite a weak one.
Of course, the introduction of Jamie Kilian may have been a great impediment in their relationship, as he continuously pursued Mitsy. This eventually brought out Hartley's paranoia and obsession with Mitsy, as he began first following Jamie, and then Mitsy during the nights. Jamie's attraction to Mitsy became apparent at Hartley's party aboard the Ida Penrose, when Jamie says "What a corker!" , in reference to Mitsy. This ignited a new rivalry between the newly formed friends, which is what Hartley disliked about Jamie most. All these things (the feelings mismatch, the particular feelings at the aforementioned turning points, the introduction of Jamie Kilian into the crew) could contribute to the weakness (or the extinct state) of their relationship. The Divine Wind (Garry Dishes).