One element of significance attributed to the character of Diana is that she accentuates Ramsay's isolation. Her character allows the author to emphasize that Ramsays' isolation is at least partly of his own choosing.
Diana is Ramsay's nurse during his long period of unconsciousness during the war. After he regains consciousness, they become very attached to each with Diana expecting that they will marry. Ramsay declines marriage after Diana brings it up stating as his reason his reluctance to be encumbered with a second mother-figure after having been newly liberated from his original mother-figure through the death of his mother. Diana associates Ramsay with Saint Dunstan (at one time Archbishop of Canterbury) who restored celibate monastic life to England in the 900s and who was legendary because of tales about his defeat of the Devil.
Thus Diana is significant because she illustrates that Ramsay is isolated by choice as much as by circumstances. Diana's character is also significant because she illustrates Ramsay's role as the Fifth Business, the baritone in European opera who facilitates the plot but who has no role outside that of facilitator and confidant for the principal characters; the Fifth never gets the girl and seemingly never notices.
"But you cannot make a plot work without ... a baritone, and he is called ... Fifth Business because he is the odd man out, the person who has no opposite .... And you must have Fifth Business because he is the one who knows the [secrets]...."