Sen. John Kerry on the death of Kara Kennedy

Sen. John Kerry (D-Mass.) reacted Saturday evening to the unexpected death of a member of the iconic Massachusetts family, the Kennedys.

Kerry watched Kara, the oldest child and only daughter of former Sen. Ted Kennedy (D-Mass.), grow up. He described her as “unbelievably proud and caring about her family, her father and her brothers, and just a very, very nice, wonderfully warm, generous person.”

She was a "brilliant mother" to her two children, Kerry told New England Cable News.

Kennedy died unexpectedly Friday after a long battle with lung cancer. She was 51.

"I watched her go through incredible sadness and incredible joy and now thought she was really on a terrific track, having been through a very difficult period in her life," Kerry said.

Kennedy, whose cancer was in remission at the time of her death, died of a heart attack after her regular, daily workout at a Washington, D.C. gym.

Kerry has been friends with the Kennedy family for years, serving with Ted Kennedy as part of the Massachusetts delegation until Kennedy’s death in 2009.