The Duke of Norfolk in this book is called Thomas Howard, and he, significantly is Anne Boleyn's uncle. He is therefore part of the faction that tries to get Henry VIII to divorce Katherine of Aragon and marry Anne Boleyn because of the power that it will bring him and Anne's family. He is first introduced in the second chapter of this work, entitled "Paternity," as making an attempt to deliberately smear the character of Wolsey, who opposes Henry's divorce and marriage to Anne Boleyn. Note what Wolsey says about the Duke of Norfolk and of what he is accused:
His Grace the Duke of Norfolk is complaining I have raised an evil spirit and directed it to follow him about... if anyone mentions it to you... just deny it.
Clearly the Duke of Norfolk is doing anything he can to promote Anne Boleyn's position and to darken the name of those who oppose Henry's divorce, even if it means accusing Cardinal Wolsey of witchcraft and black magic. The Duke of Norfolk therefore is a powerful political player in this book whose agenda is clearly to see Anne Boleyn on the throne as Queen. This of course puts him in the position of enemy to certain characters.