Who is Mr. and Mrs. Rubelle from The Woman in White and what is their importance?

Mr. and Mrs. Rubelle help Count Fosco at Blackwater. Mrs. Rubelle impersonates a nurse to watch over Marian. She also helps Count Fosco move Marian into an unused wing of Blackwater Park, so they can convince Laura that she has gone to London. Mr. Rubelle helps Count Fosco get his papers and finances in order. He also blocks Walter from leaving while the Foscos flee.