Dunstan can be seen to act as "fifth business" to Diana Marfleet, but I don't think that's a really useful way of thinking about it. If you do use that template, Dunstan is the "fifth" to her because he was engaged to her but didn't marry her. They had a...
Dunstan can be seen to act as "fifth business" to Diana Marfleet, but I don't think that's a really useful way of thinking about it. If you do use that template, Dunstan is the "fifth" to her because he was engaged to her but didn't marry her. They had a happy engagement and a fulfilling sexual relationship when Dunstan was in England recuperating from his injuries in the war, but ultimately Dunstan decided not to marry her.
Dunstan chose not to marry Diana because he saw too much of his mother in her (it could be argued that Dunstan saw too much of his mother in all women except Leisl, actually). Diana was his nurse when he was recuperating, and, though she seemed not to mind the loss of his leg or his weak arm, he saw too much of the caring and mothering instincts in her. Or at least that was what he told himself; Dunstan doesn't feel that he is cut out for monogamy.
If he is "fifth business" to Diana, it is because he was a merelly a phase in her life, and not ever her husband. He, perhaps, sharpened her ideas of what she really wanted in a husband. He remained strangely friendly with her after their engagement broke up, which does fit in with being the "fifth" -- if we must count the other four, they would be Diana, her eventual husband, and her parents.
As for the development of Dunstan as a character in regard to Diana, I think that there is no doubt that, when a woman who had everything he could want in a wife (including an equal for his intellect, which Leola Cruikshank was not) came into his life, he turned away. Though he loved her, he was not willing to marry her. He went on to lead his curious bachelor schoolmaster lifestyle, which left him free to roam the world and write his hagiography. Perhaps that is what Diana contributed to his development: the knowledge that he didn't want a wife, and saw himself, truly, as the "fifth" and never part of a couple.