Task: Question Answering
Input (English): Passage: After a row, Ellinor Devine reveals to her husband Sir Richard that he is not actually the father of their son, also named Richard, but that he was fathered by her cousin, Lord Bellasis. Sir Richard throws his son out and storms off in a rage. Shortly afterwards, Richard Junior finds his biological father dead in the forest. Only the viewer and an unidentified witness know that Lord Bellasis has actually been killed by his own son, known as John Rex. However, it is Richard Devine who is found next to the body and arrested. Thinking that his father killed Bellasis, Richard wants to protect his mother's reputation and gives his name as Rufus Dawes.
The convict ship that brings Dawes to Tasmania also carries the new governor Vickers and his wife and his daughter Sylvia. The commander of the ship is a brutal man by the name of Maurice Frere. With the Vickers is a young girl, Sarah Purfoy, as a nurse to the child. However, she really is the fiancée of John Rex, convicted for forgery, and tries to help the convicts take the ship. The rebellion is led by a murderer named Gabbett. They fail when Dawes overhears their plans and manages to warn an officer while being brought to a quarantine room for the sick. Gabbett decides to claim that Dawes was the actual ringleader. 
Question: What's the true identity of the woman who helps Gabbett with his plan?
Output (English): fiancée of John Rex.