Kathleen "Kick" Agnes Kennedy Cavendish (February 20, 1920 – May 13, 1948), born Kathleen Agnes Kennedy, was the fourth child and second daughter of Joseph P. Kennedy, Sr. and Rose Kennedy. She was a sister of U.S. President John F. Kennedy and widow of the heir to the Dukedom of Devonshire.
When President Franklin D. Roosevelt appointed Joseph Kennedy ambassador to the United Kingdom, his daughter Kathleen (nicknamed "Kick" by her family) spent a year and a half living in London. She was educated in London at Queen's College. Beautiful and spirited, she was named the "most exciting debutante of 1938". In 1943, she returned to England to work in a center for servicemen set up by the Red Cross. Despite the opposition of her intensely Catholic mother, Kennedy married William Cavendish, Marquess of Hartington, an Anglican and the eldest son and heir of the 10th Duke of Devonshire on May 6, 1944. Her eldest brother Joseph P. Kennedy, Jr. was the only member of the Kennedy family to attend the ceremony. Her husband was killed in action only four months later in World War II, and his younger brother Lord Andrew Cavendish, married to Deborah Mitford of the famous Mitford sisters, became the heir to the dukedom.
Popular on the London social circuit and admired by many for her high spirits, though more traditional members of British society found fault with her boisterousness, Lady Hartington eventually became romantically involved with Peter Wentworth-FitzWilliam, 8th Earl FitzWilliam. The couple planned to wed after Fitzwilliam's divorce. Instead, while on a trip to the south of France, Lord FitzWilliam and Lady Hartington died in an airplane crash in Saint-Bauzile, Ardèche, France.
Only her father represented the Kennedy family at her funeral. Her mother declined to attend, supposedly because of Lady Hartington's intention to marry, again, outside the Catholic Church and to a (planning to be) divorced man. It is also said that Rose Kennedy discouraged Lady Hartington's siblings from attending for the same reason. She apparently forgave her daughter not long thereafter, and in 1951, she was reportedly delighted that her first grandchild, Robert F. Kennedy's daughter, Kathleen Hartington Kennedy, was named after her late daughter, but the family requested that the child not be nicknamed "Kick". However, Kathleen Alexandra Kennedy (b. 1988), daughter of Robert F. Kennedy, Jr., does share with her late grandaunt's "Kick" nickname.
The Marchioness of Hartington is buried in the Cavendish family plot at Saint Peter's Church, Edensor, near Chatsworth in Derbyshire, England. Among the wreaths that covered her coffin was one with a handwritten note from Sir Winston Churchill. The gymnasium at Manhattanville College is named in her honor.