Katherine Seymour, Countess of Hertford (25 August 1540 – 26 January 1568), born Lady Katherine Grey, was the younger sister of Lady Jane "Nine Days Queen" Grey. Following the execution of her older sister, Katherine was seen as a potential new heir to the throne/a royal rival to both Queen Mary I and Elizabeth I. But Katherine didn't care about all of that, she was all about her secret sexy marriage to Ned Seymour, which would prove her undoing. Also, she had a pet monkey.
References: 
Elizabeth’s Women: Friends, Rivals, and Foes Who Shaped the Virgin Queen by Tracy Borman
Crown of Blood: The Deadly Inheritance of Lady Jane Grey by Nicola Tallis
Devices and Desires: Bess of Hardwick and the Building of Elizabethan England by Kate Hubbard
The Betrayal of Mary, Queen of Scots: Elizabeth I and her Greatest Rival by Kate Williams
The Sisters Who Would Be Queen: Mary, Katherine, and Lady Jane Grey: A Tudor Tragedy by Leanda de Lisle
https://www.britain-magazine.com/carousel/tudor-of-the-month-katherine-grey/
https://tudortimes.co.uk/guest-articles/love-and-loss-lady-katherine-grey
https://englishhistoryauthors.blogspot.com/2012/03/born-in-tower-crimes-of-lady-katherine.html 
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