• Kim Jong Un's sister attacked a defense agreement between South Korea and the US.
• Kim Yo Jong criticized the Biden administration, calling the president an "old man with no future."
• Kim is a powerful figure in North Korea and plays a key role in her brother's regime.

Influential North Korean figure Kim Yo Jong — the sister of the country's dictator, Kim Jong Un — insulted President Joe Biden after sharply criticizing a recent agreement between the US and South Korea to deter North Korean aggression. 

Kim commented on the agreement for the first time on Saturday, saying it would only worsen matters, according to state media KCNA. As a result of the agreement, the region is "exposed to more serious danger," she said, the BBC reported. 

North Korea is convinced it must further perfect a "nuclear war deterrent" as a result, Kim said without elaborating, according to Reuters. 

On Wednesday, Biden said at a press conference that any North Korean nuclear attack on the US or South Korea would mean "the end of whatever regime" launched it. 

Kim responded to the remark by saying it was "too miscalculating and irresponsibly brave," according to The Guardian. 

She took aim at 80-year-old Biden's age, saying, "It may be taken as a nonsensical remark from the person in his dotage who is not at all capable of taking the responsibility for security and the future of the US," the South Korean news agency Yonhap reported. 

She referred to Biden as an "old man with no future," per The Guardian. 

Kim Jong Un's sister is considered his right-hand woman and crucial to his administration.

Talks were held between Biden and South Korean President Yoon Suk Yeol earlier this week to strengthen the allies' cooperation amid anxiety over advancing weapons programs in North Korea, per Reuters.

The US agreed to send nuclear submarines to the country and involve Seoul in planning to deter nuclear threats from their northern neighbors. In return, South Korea reaffirmed it would not develop any nuclear weapons of its own.