SEOUL, South Korea — He has never traveled abroad as North Korea’s supreme leader. Until this week, his close encounters with foreign guests had been limited to dignitaries from China, Cuba and Syria — and Dennis Rodman. He increasingly resembles his grandfather, a national deity, down to the coifed flattop, gregarious grin and rotund waist.

So when a delegation of South Korean officials arrived to visit they did not know what to expect from the leader, Kim Jong-un, a 34-year-old with a nuclear arsenal, who has remained an enigma even as his weapons tests have terrified the world.

The envoys, some old enough to be Mr. Kim’s father, were taken aback by his friendliness and “forthcoming and daring” responses during a Monday meeting in Pyongyang, the North’s capital, that exceeded four hours, according to South Korean officials.

They had worried that Mr. Kim would threaten a fragile détente if South Korea and the United States resumed joint military exercises next month. Previously, the North has responded to such drills with missile tests and shrill warnings of a nuclear strike on America.