Kim Yo-Jong, believed to be 26, accompanied her elder brother to a polling station yesterday when North Korea held stage-managed elections to its rubber stamp parliament.
It was not her first appearance. She was shown on state television in 2011, tearfully standing next to Kim Jong-Un as they attended the funeral of their father and former ruler Kim Jong-Il.
Since then she has occasionally been seen accompanying her brother on his "field guidance trips".
State TV footage showed Kim in a black skirt suit, walking closely behind her brother and casting her vote into a ballot box.
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Her precise position was not detailed, but she is believed to be the events director in Kim Jong-Un's Secretariat Office.
In 2012, Kim Yo-Jong was seen on state TV riding a white horse - a common propaganda symbol associated with the Kim family - with her aunt, Kim Kyong-Hui.
Together with her husband, Jang Song-Thaek, she was seen as the power behind Kim Jong-Un's throne, until the young leader had Jang purged and executed last year.
Ahn Chan-Il, head of Seoul-based World Institute for North Korea Studies, said Kim Yo-Jong was being groomed to play the same supporting role as her aunt.
"Kim Jong-Un and Kim Yo-Jong will work in a similar way as their father and Kim Kyong-Hui did in securing the future of the Kim dynasty," Ahn said.
Kim Kyong-Hui, 67, has barely been seen in recent years, with reports that she was seriously ill and had sought hospital treatment overseas.
The Kim family has ruled the country for more than six decades with an iron first wrapped in a pervasive personality cult.