By Philippa Fogarty
Mr Chang has survived being purged to reach the heart of the leadership
Earlier this month Chang Song-taek was named as a member of North Korea's powerful National Defence Commission (NDC).
The move was seen as highly significant. It confirmed Mr Chang's position at the heart of the communist country's leadership.
Experts believe it could also indicate that contingency plans are being put in place for the eventual departure of leader Kim Jong-il.
The NDC is North Korea's highest military body. Mr Chang, a 63-year-old party veteran and administrator, is Mr Kim's brother-in-law and also his most frequent and trusted companion.
Analysts believe that should the ailing leader become incapacitated, the plan could be for Mr Chang to help steer an NDC-based collective government until Mr Kim's successor is named.
Who that successor will be remains unclear. But a hereditary succession involving one of Mr Kim's three sons is the strongest possibility.
"By promoting [Mr Chang] officially, I think Chairman Kim needed to strike a kind of balance between the military top brass and civilian leaders, which will create the best possible opportunity for his son's succession," said Dr Key-young Son, of the University of Sheffield.
Chang Song-thaek has overcome a number of setbacks to secure this top position.
When the reportedly charismatic young man met Mr Kim's younger sister, Kyung-hee, at university, the two began a relationship.
Late president Kim Il-sung was against the union because the two came from different social cliques, and he forced Mr Chang to change universities.
But he gave in after his daughter pleaded with him and the two were allowed to marry. They had one child, a daughter, who is said to have died three years ago.
Mr Chang joined the administrative ranks of the Korean Workers Party (KWP) in the early 1970s and climbed steadily. In 1992, he was elected to the party's Central Committee.
A decade later he was firmly ensconced in the upper echelons of power, as a director of a department that oversaw all other government and military departments within the party.
At the time he was seen as one of the most powerful figures in the country. But his fortunes changed and in mid-2004, despite his place in the Kim family, he disappeared from public view.
One report, citing South Korean intelligence, said he was under house arrest in Pyongyang. Others suggested he had been sent for "re-education".
It is not clear what caused his removal from power, although analysts speculated that he had gathered too much influence.
Leader Kim Jong-il is looking frail after his reported stroke last year
Either way, he did not reappear until January 2006. Since then, however, his rehabilitation appears to have been rapid.
In late 2007 he became the head of a party department overseeing police and the judiciary.
State media has increasingly reported his presence at Kim Jong-il's side on official visits around the country.
A spokesman for South Korea's Unification Ministry said that Mr Chang had accompanied him on 14 visits in 2008 - and on 19 within the first three months of 2009 alone.
"Among (Mr Kim's) entourage members, director Chang is the one who has not been seen before but is seen frequently now," spokesman Kim Ho-nyon said.
Dr James Hoare, Britain's former charge d'affaires in Pyongyang, describes Mr Chang as "more of a man in the shadows".
But it does appear, he says, that the leader's brother-in-law is being made far more prominent.
"Certainly it was said that when Kim was incapacitated with a stroke last year that Chang was being much more active," he said.
Mr Chang's appointment to the NDC cements his role as Kim Jong-il's trusted ally. It also changes - and strengthens - the military body.
Mr Chang is "a party person, not a military official - an administrator or a management figure," said Dr Daniel Pinkston, a Seoul-based analyst for International Crisis Group.
When he was appointed, the NDC was expanded from eight to 13 members, and other non-military officials were also brought in.
There is speculation, Dr Pinkston says, that this means the NDC could take on a wider administrative role, rather than looking only at military affairs.
Media reports from South Korea suggest that if Mr Kim were off the scene and one of his sons named as successor, Mr Chang would oversee him from a leadership rooted in the expanded NDC.
"It is possible that Mr Chang could work together with the son - if he is considered too young or inexperienced - as some kind of guardian or partner," Dr Pinkston says.
But the system, he stresses, remains opaque; nothing has been formally agreed.
What can be said with certainty, however, is that Chang Song-taek has reached the highest point of his career.
With Kim Jong-il increasingly frail and the country's ties with both the West and its neighbours ever more fraught, the role the North Korean leader's closest confidant is playing will be keenly watched.