In Part Two of Fifth Business, "I Am Born Again," Diana Marfleet plays the roles of Dunstan's lover, confidante, nurse / caregiver, as a hero worshipper, and as a fiance.
Firstly Diana is the nurse who cares for and nurses Dunstan back to full health after his heroic actions in the war in the Third Battle of Ypres (75-6).
Although Dunstan didn't "love" Diana, he was "beglamoured by her and flattered by her attention" (81). Shortly thereafter, she becomes his lover (82).
Diana was a confidante because they often conversed until late in the evenings (89). Diana wanted to become Dunstan's fiance (89), but they ended up being just good friends after a memorable Christmas day (90).
The main reason Dunstan and Diana don't get married, he recollects, is that "she was too much a mother to me" (85). Since he had already had and lost his own mother, he was in no hurry to acquire another. Besides, the Oedipal insinuations would be too much for him to deal with.