I think that one could find many qualities in Ratan that are endearing. For me, it would be her loyalty. I don't think it is possible to find a more loyal character than Ratan. She is an orphan, and immediately takes to the postmaster without questioning. She tends to him when he is ill, helps him with the housework and cooking, and hangs onto his every word. Ratan learns to read and write because of him. She has no desire to parlay this talent into a job or some type of career for herself. Rather, she simply wishes to be near the postmaster and in gaining literacy, she realizes that it will please him.
Naturally, her loyalty is evident at the end of the story. Her request to go with him being rebuked, the next day he finds that she has drawn a bath for him and has not forsaken her duties towards him. While any other character would have been within her own rights to leave him or be angry, she is still loyal and this is what makes her so endearing. She refuses to take money from him and cannot bring herself to even say farewell to him because of her loyalty to him. The painful ending is with her thinking that her "Dada babu" will return, as she roams the village with this image pressed to her mind's eye. It is this notion of someone waiting, someone who never foregoes the ability to wait for another who will not return that makes her so endearing and such a powerful and compelling figure in the story.