The answer, of course, is her husband of 68 years, Prince Philip. Now, as the monarch prepares to celebrate her 90th birthday on April 21, she leans more heavily than ever on her lifelong partner.
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Speaking to PEOPLE in this week's issue, Lady Pamela Hicks – a bridesmaid at the couple's 1947 wedding – says, "I can think of very few couples as united as they are."
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Princess Elizabeth was just a teenager when she fell in love with the Greek-born Royal Navy officer, whom Hicks describes as "every girl's dream Viking prince."
For more on the Queen and Prince Philip, pick up this week's issue of PEOPLE
WATCH: 90 and Still the Boss! Breaking Down Queen Elizabeth's Five Favorite Things!
Today, the nonagenarian couple (Philip will be 95 on June 10) enjoy their favorite shared interest together: collecting wildlife art, picnics in the Scottish heather and grilling with a custom barbecue cart designed by Philip complete with velvet-lined drawers for their favorite liquor. (Gin-and-Dubonnet cocktails for her, of course.)
Says Robert Hardman, writer for the recent ITV documentary Our Queen at 90, "it's the longest royal marriage in history. They don't have anything left to prove."