The chef, who goes by the pseudonym Kenji Fujimoto, described working for the Kim family in two books. The young Mr. Kim knew all about the hostility between his homeland and Mr. Fujimoto’s, from decades of Japanese colonial rule.

“When Prince Jong-un shook hands with me, he fixed me with a vicious look,” Mr. Fujimoto wrote in a 2003 memoir describing his first encounter with Mr. Kim, then 7, who was dressed in a military uniform and called “prince” by his father’s aides. “I still cannot forget the look in his eyes. It seemed to say: ‘This is a despicable Japanese.’”

He said the boy was his father’s favorite among six children.

After years of working for the family, Mr. Fujimoto fled home to Japan, where he lived in fear that North Korean agents would punish him for disloyalty. But in 2012, he returned to the North Korean capital, Pyongyang, for a visit at the request of Mr. Kim, who was now the country’s leader. The chef returned to Japan two weeks later with a positive review.

“The comrade general smiled and told me: ‘Your betrayal is now forgotten,’” Mr. Fujimoto said in a 2012 interview. He claimed the quality of life for average citizens had improved under Mr. Kim.

Mr. Fujimoto said he had been surprised at the prominent role of Mr. Kim’s wife, Ri Sol-ju, during a lunch meeting, a stark departure from his father’s style. Mr. Fujimoto said Mr. Kim was relaxed and cheerful, but also clearly in charge. The chef said Mr. Kim also seemed open to the idea of change in his country as they chatted over glasses of Bordeaux and smoked French cigarettes.