Who is Mary Bickerstaff from Sharpe's Tiger and what is their importance?

Mary is Sharpe's lover in the first part of the story. She is widowed, having been married to a British sergeant. She encourages Sharpe to seek out a promotion to sergeant and wants to marry him, but the marriage never happens. She accompanies Sharpe and Lawford on the mission and is captured with them. While she works in an enemy general's house, she decides to marry Kunwar Singh.