Just a few months ago, North Korea was regarded as nothing but a menace, rattling the region with nuclear and missile tests and staging bloody political purges at home. Almost overnight, with friendly smiles and messages of reconciliation, Ms. Kim managed to help soften her country’s image among South Koreans, at least for the moment.

She delivered her brother’s surprise invitation for President Moon Jae-in of South Korea to visit the North for a summit meeting, and Mr. Moon met her four times during her three-day trip. She held her chin up when she met political leaders and faced crowds in the South.

Her light makeup and modest, even prim, clothes were a contrast to those of her fashionably dressed sister-in-law, Ri Sol-ju. “I can’t speak very well in public,” Ms. Kim said “shyly” when she was asked to give a toast during a dinner at a five-star hotel in Seoul, according to South Korean officials who were present.

Mr. Kim “expressed satisfaction” after Ms. Kim briefed him on Monday about her trip to the South.

“It is important to continue making good results by further livening up the warm climate of reconciliation and dialogue created by the strong desire and common will of the North and the South with the Winter Olympics as a momentum,” Mr. Kim said, according to the North’s official Korean Central News Agency on Tuesday.

Despite the intense curiosity her visit generated, little is known about Ms. Kim, a member of the most secretive ruling dynasty in the world. Outside officials are not even sure about her age or marital status, though she is most often said to be 30 and married.