Long seen as one of her brother's most trusted lieutenants, Kim Yo Jong has now been elevated to a key position in the nation's ruling party.

State media said Tuesday she has been promoted to department director within the central committee of the Workers' Party -- a sign of her clout within Pyongyang.

Little is known about Kim Yo Jong -- believed to be in her late 30s -- given her prominent role in North Korea's dealings with the outside world.

Born in the late 1980s, according to the South Korean government, she is one of three children born to Kim's father and predecessor, Kim Jong Il, and his third known partner, former dancer Ko Yong Hui.

She was educated in Switzerland alongside Kim Jong Un, and rose rapidly up the ranks once he inherited power after their father's death in 2011.

North Korea frequently uses her name to issue statements outlining foreign policy or criticising Seoul and Washington.

She has effectively become Pyongyang's official mouthpiece for diplomacy with two nations typically viewed as "enemies".

"Kim Yo Jong is one of the very few people Kim Jong Un can trust and rely on," said Ahn Chan-il, a researcher originally from North Korea.

"She also served as a working-level official for Kim's summits with Trump in Singapore and Hanoi. She is experienced and seasoned," he told AFP.

Kim Yo Jong burst onto the international scene in 2018, when she visited South Korea for the Pyeongchang Winter Olympics.

- Family ties -

Every detail of her Olympic visit was closely scrutinised, from the clothes she wore to the bag she was carrying and even her handwriting.

The titular leader of North Korea's Olympic delegation -- also the country's ceremonial head of state at the time -- offered Yo Jong the seat of honour when they arrived in Seoul for brief talks with South Korean officials.

It was seen by many as a sign of her status.

Yo Jong's latest advancement within the party "amounts to promotion to ministerial rank," said Lim Eul-chul from the Institute for Far Eastern Studies at Kyungnam University.

"It's a step that is a clear show of (Kim Jong Un's) support and empowerment for her."

The Workers' Party congress offers a rare glimpse into the political workings of reclusive North Korea, and is widely seen as a forum for Kim to flex his grip on power.

There is keen interest in whether the congress might also promote another woman close to leader Kim Jong Un -- his teenage daughter Ju Ae.

North Korea's internal politics are notoriously ruthless.

Kim Jong Un executed his uncle, Jang Song-thaek, in 2013 after he was accused of plotting to overthrow his nephew.

The future of Kim Yo Jong and her career "may likely depend on the kind of relationship she builds over the next five years with Kim Ju Ae," analyst Ahn said.