"Like Jack, nothing stopped her. She was his psychological twin," says author Barbara Leaming, whose new biography, Kick Kennedy, offers a fresh look at the beloved but rebellious Kennedy daughter, who took Britain's stodgy aristocratic world by storm when she arrived in London with her family in 1938.
"Kick was as aggressive, as ambitious, as eager for power [as her brother]," Leaming adds. "This was an 18-year-old girl who desperately wanted to be a person of influence."
While both Kennedy siblings possessed great ambition, their goals were very different. "Jack sought [power] as president of the United States. And what Kick did was walk into a world which was utterly different from anything that you would expect from a little girl, Joe Kennedy's daughter from Boston," Leaming explains. "She went for [power] in the world of the British aristocracy."
For much more on Kathleen "Kick" Kennedy, the "rebel Kennedy", pick up a copy of this week's PEOPLE magazine, on newsstands Friday.
When Kick's father, Joseph P. Kennedy, was appointed U.S. ambassador to Great Britain, she traveled with him and her mother, Rose, to London, where she was presented at Court and named Debutante of the Year. What she lacked in looks and intellect, she more than made up for in wit and charm, and she captivated many of London's rich and eligible bachelors – including Billy Hartington, the future Duke of Devonshire – and a Protestant.
Against the wishes of Kick's strict Catholic family, she and Billy married in 1944 in a civil ceremony. But the newlyweds' happiness was short-lived. Just four months after they were wed, Billy was killed in action in World War II. Kick was devastated by the loss of the man she'd given up everything for. And she never got the chance to fulfill her dream of becoming a duchess.
It was "a heartbreaking love story," Leaming says. "But it's also a big piece of history because she walked in at the very moment when the power of the British aristocracy was vanishing forever, and she staked everything on becoming a duchess in a world that was in the process of changing forever."
Less than two years after Billy's death, Kick began an affair with Earl Peter Wentworth Fitzwilliam, a married drinker and gambler who was even richer than Billy. It was with Peter that Kick would meet her own tragic death, when the two lovers were killed in a plane crash in 1948.
"Because she was only 28, we don't know what would have happened to her," Leaming says. "For her family, Kick was the source of tremendous joy. After she was gone, much of that joy was extinguished."
Reporting by LIZ MCNEIL