Katerina Cranmer is the illegal wife of Archbishop of Canterbury Thomas Cranmer during Season Two. A radical Protestant reformer from the Free City of Nuremberg (in the Holy Roman Empire, a.k.a. Germany), she encouraged her husband to pursue further religious reforms in England.
Cranmer was briefly made holy envoy of King Henry VIII to the court of Holy Roman Emperor Charles V at Vienna, but encouraged by his friend (and fellow Protestant) Thomas Cromwell, he also paid a visit to the free city of Nuremberg, which was run by a radical Lutheran government. While there, Cranmer met and fell in love with Katerina; as Nuremberg laws did not enforce clerical celibacy, they married, and Cranmer smuggled her back to England in a giant crate when King Henry recalled him in episode 2.03.
In episode 2.07, Katerina attends luncheon with her husband and Cromwell at Cromwell's house; Cromwell proudly fills her in on the reforms that he and Cranmer have put in place. Katerina congratulates them, but remarks that she does not think they are changing things fast enough or far enough, especially compared to Lutherans in her native Germany. Cromwell is intrigued by her radicalism, to which Cranmer makes a joke about her illegality; Katerina, not amused, remarks that she is tired of being transported around London in a box due to that fact.
Before Cranmer can apologize, she launches into a tirade against the Catholic Church and its' hypocrisy, corruption, and inequality for women. Startled, Cromwell hastily remarks that he agrees with her, to which she replies that if that is true, he should smash and destroy every example of Catholicism he can find in England. She then turns to her husband and says, mockingly, "Now, you can put me back in my box."
Cromwell, inspired by her words, soon implements the Dissolution of the Monasteries in order to remove the power of the Catholic Church in England. However, in season Three, the King orders a return to mostly-Catholic doctrine, which Cromwell and Richard Riche note forces Cranmer to send his wife and son back to Germany or be declared a heretic. In fact, Cranmer's wife and son remained in England until the reign of the Catholic Mary I, during which they fled to Nuremberg.