Sunday's test of what appeared to be a powerful, full-fledged thermonuclear bomb, marked yet another watershed in Kim's relentless drive as leader to turn the North into a credible -- and feared -- nuclear-armed state.
In the process, he has simply shrugged off international warnings and economic sanctions, as well as bellicose threats from US President Donald Trump of possible military strikes if he persists.
When he took over from his late father Kim Jong-Il nearly six years ago, the younger Kim was in his late 20s, considered untested, vulnerable and likely to be manipulated by senior figures.
But he swiftly proved his mettle in dealing harshly -- sometimes brutally -- with any sign of dissent, even at the highest levels, while maintaining an aggressively provocative stance with the global community.
In 2013 he had influential uncle and mentor, Jang Song-Thaek, executed for treason, while he was also believed to be behind the dramatic assassination of his exiled half-brother Kim Jong-Nam in Malaysia earlier this year.
He has even shown willing to alienate the North's sole major ally China with his unstinting efforts to advance the country's nuclear and missile programmes in the face of Beijing's clear opposition -- and has still not visited his neighbour to pay his respects to leader Xi Jinping.
After his father's death, the young Kim was expected to initially rely on a coterie of powerful aides.
But that expected tutelage was short-lived as Kim started to remove any potential challenges to his authority by executing Jang.
Other purges of high-ranking officials followed and last year Kim had himself appointed as chairman of a new supreme governing commission, underlining his absolute control over every aspect of state policy.
Kim Jong-Un was born to his father's third wife, Japan-born ethnic-Korean dancer Ko Yong-Hi, who is believed to have died of breast cancer in 2004.
He was sent to school in Switzerland, where he was looked after by his maternal aunt Ko Yong-Suk and her husband.
School staff and friends, who were reportedly unaware that he was a member of North Korea's ruling family, remembered him as a shy boy who liked skiing and Hollywood tough guy Jean-Claude Van Damme.
In an interview with the Washington Post, Ko Yong-Suk -- who defected to the US in 1998 -- described Kim as short-tempered and lacking in tolerance.
He was also basketball mad, according to Ko, and even slept with a basketball.
Former Chicago Bull Dennis Rodman, who has visited Pyongyang several times, is one of the few Westerners ever known to have met Kim in recent times.
Kim knew from his eighth birthday that he would become North Korea's leader, but he only entered the public eye in 2008 when his father suffered a stroke and Pyongyang revved up plans for the nation's second dynastic succession.
By contrast, Kim Jong-Il had been publicly groomed for decades before taking over following the death of his father -- founder leader Kim Il-Sung -- in 1994.
Despite his inexperience, Kim Jong-Un has shown himself adept at the high-risk strategy of diplomatic brinkmanship practised by his father and grandfather -- engineering a series of crises and then sharply driving up the stakes and challenging the international community to respond.
Kim Il-Sung is still widely revered in North Korea, something his grandson has sought to play on by appearing to mimic his hairstyle, dress, mannerisms and public speaking style -- even his handwriting.
In the process, he has also distanced himself from the tainted legacy of his father who oversaw a devastating famine that killed hundreds of thousands -- possibly millions.
Despite being in power for nearly six years, Kim still remains something of an unknown quantity on the world stage, having never travelled overseas in any official capacity.
His personal life is also a matter of intense speculation, but shrouded in secrecy by the North.
The fact he was married was only revealed in July 2012 when pictures emerged of a young woman, Ri Sol-Ju, accompanying Kim at official events.
The couple's third child was born earlier this year, reports said last week citing Seoul's intelligence services.
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