Why did Kim Jong-un’s sister become the face of North Korea?

Kim Yo-jong, the only sister of North Korea’s supreme leader, Kim Jong-un, is now setting the tone of the country’s foreign policy. And if he is young and belligerent, she is younger and more belligerent still. Early this month, after human rights activists in South Korea used balloons to send anti-North propaganda leaflets across the border, Kim Yo-jong called them “human scum” and “mongrel dogs”. She faulted President Moon Jae-in of South Korea for failing to stop them and upbraided him for “putting his neck” into the “noose of pro-United States flunkeyism.” She warned of “action” in reprisal, apparently threatening to destroy the Inter-Korean Liaison Office set up in 2018 to facilitate...