Katharine is on her honeymoon when she meets and starts her affair with Almasy. She loves the desert, perhaps feeling it is an exotic setting where she can have an illicit love affair, defying what is proper and improper. She is the one who initiates the affair which is often violent. She breaks it off because she feels guilty and can't take the pressure of trying to keep it hidden from her husband. Her husband, Geoffrey, is an Englishman of high social standing. He's good-natured and worships his wife, bragging on her beauty to everyone. We later learn that Geoffrey is a spy for the English government sent to watch the group of desert explorers.
What does this say about Katharine? The fact that Geoffrey worships her may be too much for her to take. She doesn't wish to be a "trophy wife", so when she meets Almasy, he represents the opposite of her husband. Almasy insists that their affair does not mean that he wants a committed relationship, even though the opposite is true. He doesn't put Katharine on a pedestal to be admired for her great beauty. Perhaps this is why their affair is often violent. Almasy represents the opposite of everything Geoffrey stands for. He is fifteen years older than Katharine, so perhaps it is his age that also attracts her to him. If Katharine knows about her husband's role as a spy, she sees this as a betrayal because he's bringing the war to the desert she so loves. She feels she must betray him as well. The fact that she would start an affair while on her honeymoon tells us she is perhaps unable to make and stick with a true commitment to anyone. She ends the affair only partly out of guilt. She's also afraid of getting caught.
Hana is a twenty-year-old woman who experiences and witnesses great tragedies during the war. She refuses to accept her father's death and then must terminate her pregnancy. She idealizes the English patient, relating him to her father. She's idealistic in her attitude toward nationalism and race. She also idealizes Kip, being drawn to his brown skin. Even though Kip leaves her, she is able to achieve an emotional healing in accepting her father's death.
All of the characters in the novel are connected by love and loss and absence and desire. Katharine's love affair ends with the loss of her husband and then her own life. Her desire for Almasy is so lustful that their affair is often violent. Hana's love for her father causes her such emotional trauma that she can't accept his loss. Her desire for Kip gives her comfort and happiness for a time. Hana's father and her patient are older men who Hana idealizes. Part of Almasy's appeal to Katharine may be his older age, and at first, she may idealize him as the type of man she should have married.
Katharine and Hana are also very different. Hana is able to go from terrible grief and denial of her father's death to healing and acceptance of her father's death. It is Katharine who causes her husband terrible grief when he discovers her affair. He had idealized her, so he can't accept that she would betray him. Hana is able to commit to her ideas of nationalism, but also to her patient. She makes a committment to him when she stays behind to care for him. As I said earlier, I think Katharine has a difficult time making a commitment to anyone or anything.