How does Dunstan act as a Fifth Business towards Diana Marfleet? And how does she DEVELOP Dunstan's character?

I understand what Fifth Business means, but I do not understand how Dunstan acts as a Fifth Business to Diana Marfleet. Also, I do not understand how she DEVELOPS Dunstan's character.

I would like to know:
- how Dunstan acts as Fifth Business towards Diana
- how Diana DEVELOPS Dunstan's character in the novel

Thank you =)