A less obvious candidate for the power list as she's a little known figure, but as a prominent and well-connected woman who forwarded the ambitions of her family at court, was involved in risky secret espionage with the Scottish Court on the matter of the Stuart succession and lived openly in an adulterous relationship, having several children with her lover, a very bold move for a woman of the period, she deserves her place.
Forced into a deeply unhappy marriage aged eighteen, Penelope became the muse of Sir Philip Sidney, inspiring his sonnet cycle Astrophil and Stella and was highly influential in the Elizabethan cultural world of poets and playwrights. Her refusal to be shamed in her adultery, in which she behaved in a way that was the norm for men of the period but completely taboo for women, marks her out in my mind as a proto-feminist. Also she was listed as one of the perpetrators of her brother The Earl of Essex's rebellion and not only was she the sole woman listed but was the only person not to be tried for her part in it. This points to her having great power and influence behind the scenes.
Penelope Devereux is the heroine of my latest novel Watch the Lady (see links below)
Click for the other brilliant Tudor women in my Power List