Who is Grace Poole?

There seems to be a mystery brewing within the halls of Thornfield, the one of Grace Poole. Her name and story is always spoken in a hush tone, and everyone seems to know her history except Jane. What we do know is that she gets paid a considerable amount to work and live there, but there has been no indication of what her actual position within the house is. I have a hunch that Grace Poole and Rochester have some romantic background, whether it be just a fling or more serious, like a marriage. There is some reason as to why Grace Poole is still residing in Thornfield and maybe it is simply to protect Rochester's public image. It is possible that Grace Poole and Rochester were once married, but something went horribly wrong during their relationship and something that once ruined Rochester's image. Maybe by having Grace Poole still within his control and under his roof, he believes that his secret regrettable past could remain hidden within the shadows of Thornfield.