Known more informally as "Penny," Lady Mountbatten is not just a relative of the Queen's, but a member of her family. Lady Mountbatten's semiestranged husband is the 3rd Earl Mountbatten of Burma — and if that surname sounds familiar, it's because it's the family name of Prince Philip. Lady Mountbatten's husband traces his title to his grandfather, Louis Mountbatten, who fans of The Crown might recognise as a significant character in the lives of Queen Elizabeth and the rest of the royal family.
Lady Mountbatten's royal connection originally came from her husband, who attended the boarding school Gordonstoun with Prince Charles. However, she's become a favourite companion for the Queen as well as for Prince Philip, with whom she shares an interest in carriages.