I recently read about Lucy Percy, the countess of Carlisle, in The Man in an Iron Mask by Roger MacDonald.
What a story!
The Queen of England's closest confidant, Lucy was in fact a spy for France's Cardinal Richelieu. She was known simply as Milédi by her fellow agents.
As a teen, she was considered the most beautiful young woman in England. She eloped with one of the richest men in the kingdom, but found the younger Duke of Buckingham more to her taste. When Buckingham (supposedly) seduced the Queen of France, Lucy became a spy for France's Cardinal Richelieu.
Suspecting that Buckingham's new diamond studs were a gift from the French Queen, she managed to snip off two, intending to send them to Richelieu as evidence of the Queen's indiscretion. When Buckingham discovered the theft, he ordered the southern ports closed. I won't go into details, but suffice it to say that much skulduggery ensued on both sides of the Channel.
After Buckingham was assassinated, Lucy became the mistress of Thomas Wentworth, the future Earl of Strafford. He ended up losing his head, her hand going to John Pym, leader of the House of Commons, the man who had worked the hardest to convict her former lover.
Lucy could save lives, as well. On January 4, 1642, Charles I planned to arrest Pym and his supporters. The Queen confided in Lucy, who warned Pym.
Voluptuous, witty, but always in need of cash, Lucy journeyed to France, where she seduced the soon-to-be Musketeer, d'Artagnan. When he refused to return to England with her, he ended up in jail. Back in England, Lucy herself eventually ended up in the Tower for plotting against the English Parliament on the King's behalf.
In Dumas's The Three Musketeers, Lucy appears as Lady de Winter.
For more about Lucy, read Court Lady and Country Wife: Two Noble Sisters in Seventeenth-Century England by Rose Betcherman.
A word of caution: Macdonald is a journalist, not a historian, and he tends to play fast and free with the facts, so I cannot vouch for this account. Corrections welcome.