From The Great Successor: The Divinely Perfect Destiny of Brilliant Comrade Kim Jong Un, by Anna Fifield (PublicAffairs, 2019), Kindle pp. 245-247:
As one of the few people who Kim Jong Un trusts, Kim Yo Jong has come to play a crucial role in her brother’s regime, acting as a kind of chief of staff, protocol officer, and executive assistant all in one. She is his right-hand woman and gatekeeper.
In this way, the siblings are following the example set by their father. Kim Jong Il was very close to his younger sister, Kim Kyong Hui, the one who married Uncle Jang. He adored her, one family member would later say. After he sent his half brother into exile, she was really the only family he had. She played a crucial advisory role to her brother and held important positions within the Workers’ Party right up to her disappearance at the time her husband was executed by Kim Jong Un.
The two women were seen together at Kim Jong Un’s equestrian center at the end of 2012, both of them wearing brown jackets and riding white horses. Kim Kyong Hui appeared to be grooming her niece for the role of First Sister, just as Kim Jong Il had groomed his son.
Kim Yo Jong is several years younger than her brother; exactly how many years is anyone’s guess. The South Korean intelligence service says she was born in 1988; the US government thinks it was 1989. When she joined her older siblings in Bern, registered as Pak Mi Hyang, her birthdate was declared as April 28, 1991. That seems too late and may have been changed to get her into a younger class in Switzerland as she learned a new language.
A photo from this time shows a girl of about eight or nine with a bright smile and chubby cheeks that are a stark contrast to her angled face of today. She is wearing a choker necklace, the kind that was fashionable in the late 1990s, and a red dress. Like her mother, she loved to dance.
She led a cloistered life, growing up in the royal palaces of North Korea. Her father called her “sweet, sweet Yo Jong” and “Princess Yo Jong” and thought she was quick-witted and possessed good leadership skills. Kim Jong Il identified both Kim Jong Un and Kim Yo Jong as having an aptitude for political life.
She had joined her brothers in Switzerland and attended the same public school in Bern. She stayed there until late 2000, having completed the American equivalent of sixth grade. She is thought to have finished her schooling with a private tutor and then to have studied at Kim Il Sung University.
We didn’t see her again until it was time for her brother to take the reins. She appears in the grainy family photo taken under the tree in Wonsan in 2009, and she was at the same Workers’ Party conference in 2010 where her brother emerged as their father’s successor. She stood alongside Kim Jong Il’s fifth “wife,” who worked in the leader’s personal secretariat. This suggested that the First Sister was working in the secretariat too.
Then she was seen at her father’s funeral, a gaunt figure in a black dress, her face down as she walked behind her brother toward their father’s body. But so little was known about her that no one was sure who she was, leading to the speculation that she might be Kim Jong Un’s wife. At that stage, no one knew about First Lady Ri Sol Ju.
From the earliest days of her brother’s leadership, Kim Yo Jong has been there, supporting him.
While the glamorous Ri Sol Ju is at Kim Jong Un’s side to make him appear a more modern leader and convey a sense of aspiration, Kim Yo Jong is working. The first lady may swan about in bright outfits and clutch her husband’s arm, but the First Sister is usually seen in the background, making sure everything goes smoothly.
She could be seen popping out from behind a pillar on a balcony overlooking a huge military ceremony in Pyongyang in 2017, bringing documents to her brother that were apparently related to the spectacle taking place in the square and sky in front of them. At the opening of a flagship residential district in the capital, she was there on the stage, making sure that the photographers were in place and everything was ready before her brother arrived. She’s often checking her phone.