'Kim Jong-un knows how to be angry and how to praise. He has the ability to lead people,' Mr Fujimoto told Japanese television. 'He also loves basketball, rollerblading, snowboarding and skiing,' he said, adding that his young friend was also a fan of US basketball legend Michael Jordan.

He described Kim Jong-un as a 'chip off the old block', saying that he not only physically resembles his father but matched his tastes for fine food - shark's fin soup three times a week and plenty of tuna, washed down with cognac. 'If power is to be handed over then Jong-un is the best of it,' he said. 'He has superb physical gifts, is a big drinker and never admits defeat.'

While it may be tempting to dismiss Mr Fujimoto's recollections, there is precious little else to go on when it comes to fleshing out the life of the man at the core of one of East Asia's most important political transitions.

It looks as if the future of the world's last Stalinist regime - a hermit state that is now nuclear-armed - will one day rest upon his shoulders. What his views are about reform, much less any serious attempt to break open the doors, can only be guessed at. The only known photo of him was taken when he was 11.

Swiss journalists have tracked his childhood at the International School of Berne - a boarding school favoured by the Kim clan and other members of the tiny Pyongyang elite. The fact that it is a stone's throw from the North Korean embassy means that the gilded cage of life in the Kim dynasty was never really breached. Students and school staff described a shy boy who liked skiing and basketball and mixed with a range of children of different nationalities - including those of US diplomats - and learned French and German. The Swiss weekly L'Hebdo reported that he used the name Pak Chol and was thought to be the son of the embassy's driver.

He returned to Pyongyang at 15 - a time that put him in the orbit of Mr Fujimoto's culinary service.

Kim Jong-un remained a reclusive bit player in North Korea until a series of related events pulled his name from obscurity.

In August, his father apparently suffered a stroke that has left him frail and noticeably thinner. That brought to the fore - both inside and outside the country - questions about who would succeed him.

Speculation intensified when Kim Jong-un's name appeared on ballots in March for North Korea's parliament, the Supreme People's Assembly. While it is not known if he was chosen as a delegate, he has reportedly been appointed to the National Defence Commission - the shadowy military-dominated body that really runs North Korea.

At the same time, the country's state press has begun referring to his late mother, Kim Jong-il's third wife, Ko Yong-hi, as 'respectful mother'. The late mother of Kim Jong-un's eldest brother, the hard-living Macau habitu? Kim Jong-nam, is no longer officially acknowledged.

Then, on Monday, a week after the country's second test of a nuclear bomb, senior South Korean intelligence agents told a parliamentary committee in Seoul that Kim Jong-un had been formally anointed Kim Jong-il's eventual successor.

No statement has been made in North Korea, but the South's National Intelligence Service told parliamentarians that a personality cult was already springing up around him. Key institutions - the army, the assembly's Presidium and overseas embassies - had been ordered to sign loyalty pledges to him. Patriotic songs glorifying his image were already being practised by troupes in Pyongyang.

With no official confirmation, of course, the situation raises as many questions as answers, and there are several important caveats. South Korean intelligence is frequently derided, both inside and outside the country, as a contradiction in terms.

For years, the South has lacked solid networks of well-placed sources in the North Korean leadership. It was no surprise that, upon taking power, President Lee Myung-bak quickly ordered the intelligence services to dramatically step up spying operations against the North, specifically with a view to forging new networks. US officials, too, describe a similar lack of 'humint' - that is, human intelligence, comprising reliable people inside the leadership able to confirm and outline key events.

'We may have lots of electronic means of surveillance outside of the country, but in terms of what is going on on the ground among the leadership we're guessing,' said one US envoy with long experience of the Korean Peninsula. 'There is no other place quite as silent - in terms of meaningful leaks, their isolation is virtually complete.'

If the succession story does prove correct, it will be a rare coup for foreign intelligence agencies.

But Kim Jong-un's two siblings also have claims to the succession. His brother, Kim Jong-chul, has the same mother but has long been derided by his father as too effeminate to rule such as a militaristic country - another claim linked to the memoirs of Mr Fujimoto.

As the eldest son, 37-year-old Kim Jong-nam's bloodline is the strongest. Also Swiss-educated, Kim Jong-nam is thought to have a decidedly internationalist outlook. Keen on parties and drinking, he prefers the freedoms of cosmopolitan Macau, Bangkok and Paris to Pyongyang and even Beijing, where he has attracted the attention of Chinese officials keen to protect their stake in a smooth transition. Asian diplomatic sources describe him as sharp and witty, but far too cynical to want to lead.

He has played a key role in developing Pyongyang's cyber-warfare skills and other online capabilities, but is widely seen to have dented his chances with his arrest and deportation from Tokyo in 2001. He was caught with family members trying to get through airport immigration with a fake Dominican passport.

The role of Kim Jong-il's newly powerful brother-in-law, too, is important in the uncertain months and years ahead. Jang Song-taek is Kim Jong-un's uncle and played a key leadership role during Kim Jong-il's illness last year. Given the two-decade transition after Kim Jong-il was nominated by his father, Kim Il-sung, in 1974, it is almost certain Mr Jang is being groomed as communism's first regent. He is considered ready to rule if need be until Kim Jong-un is ready.

If Kim Jong-un has the nod, it is unlikely North Korea will confirm anything soon, given its habitual secrecy.

Like diplomats across the region, usually courteous and professional, if uninformative, North Korean consular officials in Hong Kong did not even want to take basic questions this week. 'No interview, thank you,' one said, putting the phone down before a question could even be asked.

US officials confirm that North Korean diplomats based at the United Nations in New York - a key back channel for communication - have gone silent on all matters as well.

Hanging over the transition is North Korea's intensifying nuclear programme. A growing body of opinion is taking at face value Pyongyang's bellicose claim that the six-nation process to rid it of its nuclear weapons is dead. Maybe it wants a nuclear stockpile not as a bargaining chip but for a long-term deterrent against invasion.

The regime is entering an important phase ahead of the 100th anniversary of Kim Il-sung's birth. The leadership is determined to have the transition secured, Asian and western diplomats believe, and to keep the military as strong as possible.

If Pyongyang is going to enter into any fresh talks during this time, it may want to do so only on very different terms.