Known for: a likely heir to Elizabeth I whose secret marriage incurred the wrath of Elizabeth, resulting in Catherine's heirs being passed over for the succession to the English throne
About Lady Catherine Grey:
Lady Catherine Grey was the daughter of Henry Grey, duke of Suffolk, and Lady Frances Brandon, whose mother was a sister of Henry VIII and whose father was a close friend of Henry. Catherine was the younger sister of Lady Jane Grey, England's Nine Day Queen.
After her sister was executed, Lady Catherine Grey was in line for the English throne after Mary I and the Princess Elizabeth, according to the terms of King Henry VIII's will. When Elizabeth I became the reigning queen, Lady Catherine Grey was next in line -- and thus her marriage was a matter of great concern to the crown.
In 1560 Lady Catherine Grey secretly married Edward Seymour, who had been created Baron Beauchamp and earl of Hertford by Elizabeth I in January 1559. Catherine's new husband was a nephew of Henry VIII's third wife, Jane Seymour, and son of Edward Seymour, Lord Protector under Henry VIII and Edward VI, who was executed by the Duke of Northumberland in 1552.
When Queen Elizabeth discovered the marriage of Lady Catherine Grey and Edward Seymour, she imprisoned both Catherine and Edward from 1561 until 1563. During Catherine's time in the Tower she gave birth to her first son.
Lady Catherine Grey died in 1568, and Hertford lived out of the public eye until his death in 1621.
Elizabeth refused to recognize the marriage as valid or to recognize Catherine's two sons by Hertford as legitimate. Thus, the elder, Edward Seymour, was passed over for succession when Elizabeth chose her heir on her deathbed.