Media reports have said leader Kim has anointed Kim Jong-un to take over the family dynasty and Japan's influential Asahi newspaper said on Tuesday the younger Kim secretly visited ally China last week as a special envoy of his father.
There he met Chinese leaders including President Hu Jintao while an aide to Jong-un told Chinese officials the younger Kim had been appointed heir and that he held an important post in the ruling Korean Workers' Party, the Asahi said.
Swiss-educated Jong-un, thought to be 25 and whose name in Chinese characters translates as "righteous cloud", is the youngest of Kim's three known sons.
Jong-un is thought to speak English and German, enjoys his liquor and bears a striking resemblance to his father, informed sources have been quoted in local media as saying.
South Korea's defence minister said this month the North's recent military moves including its May 25 nuclear test were aimed at helping Kim Jong-il, 67, pave the way for succession after questions of his leadership were raised when he was thought to have suffered a stroke last August.
The only known photograph of Jong-un until this week was of him as an 11-year-old. On Sunday, another Japanese daily published a photograph of him when he was 16, but his exact age remains unclear. He was born either in late 1983 or early 1984.
There is a question too over whether his late mother, a Japanese-born professional dancer called Ko Yong-hui, was Kim Jong-il's official wife or mistress – an issue that might weigh on his legitimacy to replace his father.
CLOAK OF SECRECY
Even by intensely secretive North Korean standards, remarkably little is known about the son, whose youth is also a potential problem in a society that adheres closely to the importance of seniority.
Kim Jong-il was very publicly named heir by his father, Kim Il-sung, but he has studiously avoided repeating the process.
None of his three sons are mentioned in state media, much of whose efforts are focused on eulogising the current leader and his father who died in 1994 and is the eternal president of Asia's only communist dynasty.
After taking over, Kim Jong-il has seen his state's economy grow weaker and a famine in the late 1990s kill about one million of his people, while he has advocated a military first policy.
In a book about his time as chef to the ruling household, Kenji Fujimoto of Japan said that of the three sons, the youngest Kim most resembles his father.
He is also reported to have a ruthless streak and the strongest leadership skills of the three. And, perhaps more importantly, he is thought to be his father's favourite.
Park Syung-je, a Seoul-based analyst with the Asia Strategy Institute, said he believed Kim junior had the backing of Jang Song-taek, effectively the country's number 2 leader.
Kim Jong-il in April promoted Jang, his brother-in-law, to the powerful National Defence Commission, which many analysts took to be an attempt to establish a mechanism for the eventual transfer of power, with Jang as kingmaker.
South Korean media have speculated that Kim Jong-un may also suffer diabetes, something that is thought to have long plagued his father.
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