The power behind the throne in North Korea was a dashing accordion player in his youth, whose life changed when he met the daughter of autocratic founder Kim Il-Sung at university and wooed and married her despite the dictator's opposition.
Now an ascetic-looking, bespectacled 65, Jang Song-thaek has overcome a purge, bitter palace intrigue and personal tragedy to become the chief adviser to his nephew Kim Jong-un, the third-generation leader of North Korea after his father Kim Jong-il's death last week.
While little is publicly known about North Korea's first family, acquaintances, South Koreans who met Jang during a 2002 visit and analysts say he could steer the young Kim toward opening up one of the world's most isolated states.
He did seem to be someone who's more interested in the outside of North Korea and more interested in the rest of the world, former U.S. assistant secretary of state Christopher Hill told Reuters, adding however that he needed to see more evidence.
Jang Song-thaek is interesting, added Hill, who travelled to Pyongyang as a U.S. special nuclear envoy from 2005 to 2008.
In recent pictures out of Pyongyang, Jang is one of the few men in civilian clothes seen standing near Kim Jong-un at the mausoleum where his father's body lies in state.
Jang is a leading member of the coterie that will now rule North Korea, a source with close ties to Pyongyang and Beijing told Reuters this week.
His official titles include vice chairman of the National Defense Commission, the supreme leadership council which Kim Jong-il led as head of the military state.
In Seoul, officials and other sources said Jang has three traits that make him suited to run, or at least influence the running of North Korea: he understands the mechanics of Pyongyang's power structure, he knows South Korea well, and he has good connections in North Korea's 1.2-million-strong military.
Han Kap-soo, a former South Korean agriculture minister who hosted the North Korean delegation in 2002, said Jang was diffident, but clearly not someone to be taken lightly. He was also fond of a drink.
As expected, the focus was on Jang, Han told Reuters. But he never came out in front. When we tried to take pictures, he tried to move to the back.
But one morning, after a night of hard drinking, Jang failed to emerge from his room on time. No one dared knock on his door.
I thought, 'Oh, people can't even go wake him up,' Han said.
Jang was born in 1946. He was not from the North Korean elite, and not the sort of husband Kim Il-sung wanted for his headstrong daughter Kim Kyong-hui, an author and some North Korean defectors have said.
Nevertheless, he attended the prestigious Kim Il-Sung University in Pyongyang.
He drank well and was fun and played the accordion well. And he was a very smart guy, said Jang Sung-min, author of the book War and Peace: Where is North Korea headed after Kim Jong-il.
So many girls liked him. He was so charming, that's why Kim (Kyong-hui) fell in love with him.
Joo Sung-ha, one of the defectors, said the couple, who were classmates, continued to meet despite opposition and Jang's exile to another university.
Kim Kyong-hui drove Kim Il-sung's Mercedes whenever she had time to meet Jang and did laundry for him after he moved to another university due to Kim Il-sung's opposition, said Joo, citing other North Korean sources. Joo attended Kim Il-sung University but in later years.
The two wed in 1972, but it was not a happy marriage, said Joo, who was a university lecturer in North Korea before defecting.
Kim Kyong-hui was later described as an alcoholic and the couple's daughter committed suicide at her apartment in Paris in 2006, ironically because her parents were opposed to her boyfriend.
Kyong-hui later became Kim Jong-il's most trusted confidante but often disparaged her husband, at times treating him like a subordinate or a servant, according to a Japanese chef who cooked for the leader in the 1990s.
In 2004, Jang was thrown out of the power clique because of differences with Kim Jong-il and perhaps for having close ties with the leader's disgraced first son Kim Jong-nam.
But he was rehabilitated two years later, apparently at the behest of his wife, and he later switched his support to Kim Jong-il's favoured third son, Jong-un.
He also ran afoul of other powerful people at the time and was badly hurt in a car accident, South Korean media said. It's not known how he did it, but he was soon back in the inner circle.
He is exceptional in the grasp of the mechanics of power, said Baek Seung-joo of the state-run Korea Institute for Defense Analyses. He is a powerful man, a skilled operator and a bold person, which was apparent as early as in 2000.
After his brother-in-law suffered a stroke in 2008, Jang gradually boosted his influence and cemented his place in the power circle, becoming, with his wife, one of Kim's most trusted advisers by the time of his death.
Jang also amassed influence during a long tenure in the powerful Organisation and Guidance Department of the ruling Worker's Party. In the 1990s it was believed to be the most powerful department in all of North Korea, said Lee Jong-seok, an authority on the North's power elite.
Jang is now likely to seek control of the party's Office #39, the source of financial resources to control and influence the elite, South Korean analysts say.
Office #39 would allow him to maximise domestic influence and possibly extend his reach over foreign policy and North Korea's nuclear programme, according to a 2010 U.S. Department of Defense report.
That post would give him the ability to open up North Korea if he could do so without destabilising the regime, analysts said.
I think Jang Song-thaek is the one who acknowledges the need for cooperation with the outside, said Han Ki-bum, who was a specialist at the National Intelligence Service, the South's spy agency, until 2009.
In public and on the surface, Jang prefers to keep a low profile. But that only conceals the power he wields, those who have met him say.
He was a hard drinker, with a very powerful image, and he seemed to have a sense of authority and power, said Park Jie-won, a member of parliament who met Jang, once in Pyongyang and again when he was part of a high-level delegation that toured industrial sites in the South in 2002 under warming ties.
He came across as very sharp, said Park, who was also the right-hand man of former South Korean President Kim Dae-jung, who held the first summit with Kim Jong-il in 2000 that led to a breakthrough in the bitter rivalry between the two Koreas.
But for all his power, Jang can be over-ruled by one word from his nephew. And he is unlikely to seek power for himself.
Jang Song-thaek and Kim Jong-un can't abandon each other, said Joo, the defector.
Kim Jong-un lacks solid power now and Jang sees no need to risk his life to overthrow Kim. Jang will have more say in this new Kim Jong-un era but Kim Jong-un is the one who makes the decisions.
(Additional reporting by Paul Eckert in Washington; Editing by Raju Gopalakrishnan and Dean Yates)