Kim Yo Jong's title supports earlier reports from a North Korean defector group which said she may have taken a high-level role when Kim Jong Un recently disappeared from public view for more than a month, prompting speculation about his grip on power.
South Korea's intelligence agency later said Kim, 31, was likely to have had surgery on his left ankle. Kim has since reappeared, walking with a limp.
Kim Yo Jong's power has been likened to that of a prime minister, an unnamed South Korean intelligence source told the Seoul-based JoongAng Ilbo newspaper in April, even before her brother's injury.
"All roads lead to Comrade Yo Jong," the source said.
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Kim Yo Jong has featured in state propaganda since her brother took over the nuclear-capable country upon the death of their father, Kim Jong Il, in late 2011.
In 2012, as state TV showed Kim Jong Un arriving at the opening of an amusement park in Pyongyang, Kim Yo Jong ran from one position to another between ranks of applauding party cadres and generals as if she was orchestrating the event for the new North Korean dictator.
Since then, the smartly-dressed Kim, her hair usually pulled back in a ponytail, has made several appearances with her brother, giggling at state concerts, presenting awards to fighter pilots or riding a white horse.
Women in patriarchal North Korea rarely become high-ranking officials or military commanders. They do, however, receive military training.
They are also vital to North Korea's moribund economy. With many men engaged in state-appointed jobs in factories and bureaucratic departments, it is often women who turn to black market trading to earn the income most families need to survive.
But for Kim Yo Jong, it is her family name and proximity to Kim Jong Un that supersedes any cultural norms.
"People who are nominally her superiors most likely defer to her," said Michael Madden, an expert on the North Korean leadership.
A North Korean princess
When Kim Jong Il ruled North Korea, his sister Kim Kyong Hui took a powerful role as a personal assistant with high-ranking military and party jobs.
She has not been seen since her husband, Jang Song Thaek, once regarded as the No.2 leader in Pyongyang, was purged and executed late last year.
Writing in his 2003 memoir about his 13 years as Kim Jong Il's sushi chef, Kenji Fujimoto said the late dictator had a trusting relationship with Ko Yong Hui, his fourth partner, with whom he had three children: Kim Jong Un, Kim Yo Jong, and their elder brother Kim Jong Chol.
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"Ko said she had traveled to Disneyland in Europe and Tokyo with her kids," Fujimoto wrote.
Not much is known about the elder Kim, who was once photographed at the Swiss boarding school all three children reportedly attended in a replica of Dennis Rodman's NBA basketball jersey.
Even at dinner, Fujimoto said, Kim Jong Il kept his eldest son at arm's length, preferring to place future leader Kim Jong Un and his sister, beside himself and their mother whom he called 'madam.'
"Kim Jong Il sits in the middle, and to his left, sits his madam," wrote Fujimoto.
"Prince Jong Un sits to the left of the madam, and the princess sits to the right of Kim Jong Il."