For Mr. Kim, it was something of a debut, in which he presented himself to a global audience without the protection of the state media he controls or the filter of visitors such as the basketball star Dennis Rodman, one of the few outsiders who has met him.

Shifting his weight from side to side while speaking, he projected confidence in a husky voice — a voice usually heard only in prepared speeches broadcast in the North.

[Read selected quotes from Kim Jong-un’s remarks during the historic summit meeting here.]

Many watching in South Korea saw him as a younger, more heavyset version of his grandfather, North Korea’s founder, Kim Il-sung, and some noted his heavy breathing and wondered about his health.

There were glimpses of the absolute authority he wields at home, too. Mr. Kim drove to lunch in a Mercedes limousine surrounded by 12 running bodyguards, for instance. And before he signed a guest book in the lobby of the conference hall, North Korean officials rushed to disinfect the desk and chair, as well as the book and a nearby pen. (Even after all that, Mr. Kim used a pen provided by his sister, Kim Yo-jong, the only woman in his delegation.)