Characters: Letters Unposted

At the beginning of the story, Mary is a mystery.  No one knows much about her, where she came from, does she have family, etc.  She needs a place to stay.  Henry Brown needs someone to help him cook and clean now that he has Alexander and Thomas.  She moves in, solving both of their problems.  

Like Henry, she keeps her own council and speaks rarely.  She doesn't seem interested in getting to know the house's inhabitants and goes about her duties with a minimum of discussion, and avoiding contact unless necessary.

What is her story?  Has she always been so unhappy and dispirited, or has something happened that has made her this way?  How will joining the Brown household effect her?