Lady Ursula Missledon is a noblewoman who becomes a lady-in-waiting for the new Queen, Jane Seymour, in Season Three of The Tudors. She is portrayed by English actress Charlotte Salt and appears in five episodes, leaving after the death of her mistress.
Lady Missledon arrives at court only to be immediately pursued by Sir Francis Bryan, a lecherous and unscrupulous agent under the King. Despite being aware of Bryan's reputation as a man who "boards other men's boats", Lady Missledon does not turn down his blunt offer to become his mistress despite having a fiance, Sir Richard Taverstock. Lady Missledon doesn't seem to care much for Sir Richard, willingly exploiting her relationships with Bryan and the King to gain favors.
However, King Henry - who is becoming impatient that his new wife, Jane Seymour, is not with child - also takes an interest in her and Bryan, after realizing this, passes her on to the King.
Henry has Ursula pose nude for a portrait he has commissioned to Hans Holbein; this results in a humorous but violent encounter between Holbein, Ursula and her fiance, Sir Richard Taverstock.
Jane Seymour's principal lady-in-waiting, Lady Jane Rochford, realizes Henry's infidelity and loyally reports to the Queen that Lady Missledon is the King's mistress; Jane, who makes a point of being submissive to Henry, graciously accepts the situation, since she is already pregnant with Henry's child. When Jane goes into labour, she asks Missledon to "be a comfort" to Henry if she does not survive childbirth. Feeling guilty, Lady Missledon promises Jane that she will.
When Jane dies just days after giving birth to Prince Edward, Henry spends an extended period of time grieving for his wife, but eventually returns to court. He asks Ursula what she will do now that her mistress is dead; she replies that she will go home and live a modest life.
Henry commends Ursula and releases her from being his mistress, but asks her to spend one more night with him; Ursula, later that night, reluctantly enters Henry's chambers in a thin robe. After he tears it off of her, she gives into her lust and they proceed to engage in unrestrained sex multiple times for the rest of the evening.
After 3.05, she is not seen again in the series.
While it is true that Henry had several mistresses (many of whom he later made his wife), Lady Ursula Missledon is a fictional character created for the series. Most historians believe Henry was faithful to Jane Seymour. It's possible she is based on Anne Bassett, who was rumored to have been Henry VIII's mistress after Jane's death.