Kathleen 'Kick' Kennedy.Getty Images

The story of Kathleen ‘Kick’ Kennedy, the rebellious fourth daughter of Rose and Joseph Kennedy who defied her family for love, was unpublicized after her untimely death at the age of 28.

“She was the only rebel of the family,” says Lynne McTaggart, author of 1983's "Kathleen Kennedy: Her Life and Times." “If you look at all nine [Kennedy] children, she was the only one who didn't march down the prescribed road.”

Her story was recently featured in the series "Million Dollar American Princesses" on the Smithsonian Channel, the Daily Mail reports. The documentary was narrated in part by Kick’s namesake, Robert F Kennedy Jr’s 27-year-old daughter.

“When I was little, I wondered why I had a funny name,” Kathleen 'Kick' Kennedy, 27, told the New York Post. “I was named after my great-aunt, who was a lot of fun. She was a 'kick’!” 

Robert F Kennedy Jr's 27-year-old daughter Kathleen, who was named after her great aunt. Credit: Getty Images

Robert F Kennedy Jr's 27-year-old daughter Kathleen, who was named after her great aunt. Credit: Getty Images

The elder Kick, who was born in Brookline, MA in 1920, was a charmer who never failed to catch the attention of the opposite sex.

“She dated friends of her brothers – red-blooded American jocks,” says McTaggart.

When Joe Kennedy was appointed US ambassador to the UK, he and his family departed for London. There 18-year-old Kick was named debutante of the year. 

“She was idiosyncratically charming,” says her namesake niece. “She would call the Duke of Marlborough ‘Dukie Wookie’ and chewed gum walking down the streets of London.”

At a party, she met William Cavendish, Marquess of Hartington, who was set to be the future duke of Devonshire. Kick called him Billy.

“He was a great catch, and a sweet guy,” McTaggart said, adding that he was “rather shy” compared to Kick. 

Billy and Kick soon fell in love.

However, after Germany invaded Poland in 1939, Joe Kennedy sent his family back to the United States. Although the 19-year-old Kick begged to stay so she could be with Billy, she returned to America. 

Kick stayed in the United States for the next four years, but was determined to make her way back to the UK. She joined the Red Cross, which was sending volunteers to England. 

She finally made it back to London, and she and Billy, who was in the British Army, picked up where they had left off. Rose Kennedy, however, was not happy that her daughter was with a Protestant. 

“Marrying outside of the church was probably the worst sin one could commit,” Kick Kennedy explains. “It meant living one's life in mortal sin and eventually going to hell.” 

Kick ignored her mother and married Billy in a civil ceremony in May 1944.

Kick (right) married William Cavendish, Marquess Of Hartington (left) in 1944. The only Kennedy in attendance was her brother, Joe Jr. Credit: 100% Keystone.

Kick (right) married William Cavendish, Marquess Of Hartington (left) in 1944. The only Kennedy in attendance was her brother, Joe Jr. Credit: 100% Keystone.

Kick's older brother Joe Jr was the only Kennedy in attendance. 

Four weeks later, Billy was sent to the Belgian front. 

In August of the same year, Joe Jr was killed when his plane exploded while he was over France on a secret bombing mission. 

Less than a month later – four months after Billy and Kick wed – Billy was shot through the heart by a German sniper.

“I can't imagine anything more devastating,” says the younger Kick. “But the rule is, Kennedys don't cry.”

After mourning Joe in the United States, Kick returned to England as Lady Hartington. 

She soon found love again with Peter Fitzwilliam, a wealthy Protestant who was also a married man. 

Fitzwilliam was in the process of divorcing his wife when Kick and he began a relationship, but her parents were still horrified and threatened to disown their daughter. 

“When you've seen so much tragedy during the war, it makes you feel that you'd better live for the moment,” said McTaggart. 

“Fitzwilliam had a lot of money and was a lot of fun,” she added. “I think the chances of him being faithful to her were zero.”

While traveling to France to meet up with Kick's father, the couple boarded a plane for a stop on the Riviera. Storms caused the plane to crash in the mountains and all passengers and crew were killed. 

Kick's father Joe was the only Kennedy at the funeral. 

The Kennedy family kept the death quiet due to the scandalous circumstances surrounding Kick’s death and because of JFK’s burgeoning political career.

The Kennedy family in London. From left: Edward, Jeanne, Robert, Patricia, Eunice, Kathleen, Rosemary, John F Kennedy, Rose Kennedy, Joseph Kennedy. Credit: Getty Images

The Kennedy family in London. From left: Edward, Jeanne, Robert, Patricia, Eunice, Kathleen, Rosemary, John F Kennedy, Rose Kennedy, Joseph Kennedy. Credit: Getty Images

She was buried in a small churchyard in Edenser, England as Kathleen Cavendish, Marchioness of Hartington. Her husband Billy was buried in Belgium where he was killed.

“The times she lived in necessitated bravery and a strong ability to carry on in the face of tragedy,” said the younger Kick Kennedy. “Her decisions were informed by her own moral compass, not that of her parents or society.”

She says she cherishes a photo of her great aunt dressed in a Red Cross uniform.   

“[My great-aunt] looks quite beautiful and you can feel her vigor,” she said of the photo. “I find her story powerful and her spirit-inspiring.”

* Originally published in Jan 2016.