Madame de Mauban is first introduced as the woman with whom a friend of Rassendyll's is in love. She, however, has grander aspirations and is pursuing the Duke of Strelsau. Later in the novel, it appears that her attachment is not merely ambitious, but that she really does love Black Michael, who, in his turn, only cares for the Princess Flavia.
This is Madame de Mauban's principal reason for acting as a double agent and helping Rassendyll in his impersonation of the king. She even writes to Princess Flavia to warn her not to come to Zenda at Michael's invitation. Although she recognizes that Michael is a villainous character, she wants to marry him. In order to do this, however, she must thwart his ambition to become king, since this would allow him to marry the princess. She is therefore an ally of Rassendyll and the king—though a somewhat unreliable one, since her motive is jealousy.
Although the title, The Prisoner of Zenda, clearly refers to the king, Madame de Mauban also becomes little more than a prisoner in Zenda and finally begs Rassendyll to rescue her. Despite this, she remains in love with, if not loyal to, the Duke and is last seen mourning beside his coffin.
Although The Prisoner of Zenda is a romantic story, the two love affairs that feature most prominently in it do not end well. Princess Flavia is trapped in a loveless marriage to the king, and Madame de Mauban weeps beside a coffin. Both these sacrifices were necessary for stability and civil order, and it is highly unlikely that the king would have survived or the impersonation succeeded without the help of Madame de Mauban.