I think that the basic elements of both characters is that one enters a setting with little in way of assistance and another has lived in this setting with little in way of assistance.  The Postmaster enters the village in fairly dire conditions, trying to cope with the challenges of village life.  He is educated, well versed, and very must "together" from an exterior point of view.  His location in the village is a challenging one, an assignment that he does not necessarily enjoy.  Ratan, by contrast, is an orphan, whose only world has been the village and a sense of isolation that accompanies abandonment in the process.  There is a need in both of them.  The postmaster needs a diversion, indicated by the fact that he really has never been alone in his life.  Letters from his mother indicate this, that he had been attended to and always around people.  Ratan had never experienced this and the presence of the postmaster allows her to feel a sense of need, when the two of them share conversations late into the night, or when she helps him with his various duties.  Ratan is shown to be extremely willing in terms of her state of being with the postmaster, learning to read and write in order to be closer to him and to share companionship with him.  By contrast, the postmaster is extremely uncomfortable in the village and with his work.  Tagore seems to be constructing a portrait of someone who is perceived as "higher" in social orders that is a bit inferior on a moral and ethical level to the individual perceived as "lower" in the social order.  This lack of moral stature is proven at the end when she asks to go with him and he can only scoff derisively at such a notion.