(For full coverage of North Korea, click [nN25330519])
By Jonathan Thatcher
SEOUL, May 26 (Reuters) - Hidden from even the North Korean public, the youngest son of iron ruler Kim Jong-il has for months been the focus of discussions about who might next lead the impoverished state.
Speculation over who will succeed to the world's first communist dynasty has grown after reports that Kim, who took over from his father and the country's founder in the mid-1990s, suffered a stroke last year.
Many analysts believe the North's internationally condemned nuclear test on Monday was partly aimed at boosting the 67-year-old leader's standing at home to give him more leverage in anointing an heir -- believed to be his third son, Kim Jong-un.
There is no confirmed photograph of the adult Kim Jong-un and his age is unclear. He was born either in 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.
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 now North Korea's eternal president.
Kim Jong-un is thought to be Swiss-educated and able to speak English and German.
In a book on his time as chef to the ruling household, Kenji Fujimori 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.
In a report on Monday, the Wall Street Journal said Washington had concluded Kim had initiated a political transition in which Jang and the younger Kim were emerging as major players in a new power structure.
South Korean media have speculated that Kim Jong-un may also suffer diabetes, something that is thought to have long plagued his father.
(Additional reporting by Kim Jung-hyun and Jack Kim, Editing by Dean Yates)