Mary Guise appears not as a direct character in the book but more as a constant threat to Queen Elizabeth's reign. Mary Guise is currently betrothed to the Dauphin of France, who is the heir to the French throne, and so is expected to become the next Consort-Queen of France. Mary Guise is a Catholic princess with a claim to the English throne who is being harbored by some of the noble families in Scotland before she can make an attempt at an invasion of England, backed by the French. Mary is talked about as a threat and as a horror for Protestant England, and already apparently bears the English coat of arms as a part of her banner. During the course of the book she is facing an uprising of the Protestant Scottish Lords against her rule, and this uprising is tacitly supported by William Cecil and money sent by Elizabeth's treasury.
