Greedy, violent, with an explosive temper and inclined to swagger over his classmates is the unvarnished reality that has emerged of North Korean dictator Kim Jong-un's schooldays.

The true picture supersedes an earlier, airbrushed version which had a young Jong-un as a basketball loving, American culture-obsessed teenager.

Certainly he liked basketball while attending the elite Liebefeld Steinhölzli state school near the Swiss city of Bern between 1998 and 2000.

But according to Axios.com, a classified file on the leader's formative years compiled from interviews with the school's teachers, students and staff, portrays a darker portrait of the future despot.

Jong-un was hot-tempered, a bad loser, with a ruthless determination who lorded it over other children including his older brother, once North Korea's heir apparent, Kim Jong-chul.

"The picture that emerged from literally dozens of interviews bears a striking similarity with the man he has emerged into today," an intelligence source told Axios.

"Gluttonous, prone to fits of anger and swaggering around his classmates, Kim Jong-un was an in-attendant student but demanded slavish loyalty from other children in his wake.

"He was prone to violence. He had a couple of young guys who were with him … he hit them frequently.

"He didn't do well in school. He was distracted a lot."

The US intelligence file on the reclusive Jong-un came under the spotlight due to US President Donald Trump's interest in the leader's personality traits ahead of their June summit in Singapore.

Jong-un was the middle child of Jong-Il's three children with partner Ko Yong-hui, news.com.au reported.

Older brother Kim Jong-chul, younger sister, Kim Yo-jong and Jong-un were the legitimate grandchildren of North Korea's first leader and self-proclaimed "Eternal President of the Republic" Kim Il-sung.

But he was not considered a successor, and merely third in line to take over from Jong-il, until his dominant traits emerged and his two rivals failed to impress.

Kim Jong-nam, who was fatally poisoned with a nerve agent in Kuala Lumpur airport in February last year, was considered heir apparent until 2001.

The current dictator's older illegitimate half brother fell from favour after getting caught trying to visit Disneyland without a passport, but ultimately was an advocate of reforming his country's totalitarian regime.

Hereditary rule then fell to Jong-chul, but Jong-un's brother was weak according to his father.

Jong-chul, who was photographed in his twenties attending Eric Clapton concerts and is a keen guitar player, was deemed "no good because he is like a little girl" by his father.

As children, the brothers were kept from public view and allowed to associate with only a few officials in the Jong-il's regime.

That included Japanese chef Kenji Fujimoto, hired by Jong-il to prepare sushi for the leader and his entourage.

Mr Kenji told PBS's Frontline that he met the brothers when Jong-il instructed him to be the children's playmate.

On meeting, Jong-chul offered a firm handshake, while Jong-un held back and stared, reluctantly extending his hand only at his father's insistence.

When the boys played basketball, Mr Kenji said, Jong-un would boss around Jong-chul, telling him what to do.

It was like the younger boy "was the older brother".

Mr Kenji said Jong-chul was kind to others who played basketball with the brothers, saying, "Good job everyone, have a good rest".

In contrast, Jong-un would berate players about their game.

Jong-un learned to drive at the age of seven, got his first Mercedes aged 10, and hated being treated like a child.

Mr Kenji became Jong-un's confidante, and started smoking in his early teens, Yves Saint Laurent menthol cigarettes, then the world's most expensive smoke.

The boy demanded the chef not tell his father.

Mr Kenji has described the young Jong-un as hot tempered and a sore loser, with a ruthless determination.

An example of Jong-un's incendiary temper related to his smoking was revealed by university security expert Nam Sung-wook last year.

In a story about the leader's threat top global security, The Korea Herald reported Mr Nam's claims that as a 15-year-old student in Bern, Jong-un had a girlfriend back in North Korea.

The girlfriend was a year older than him, but his explosive reaction when she suggested he quit smoking shocked her.

"When [she] asked him to quit smoking on the phone, apparently concerned with his young age at the time, Kim became furious and responded with vulgar language," Mr Nam said.

"It was very shocking back then."

As well as American basketball, Hollywood films and McDonalds dominated the young Jong-un's life.

He also demanded Mr Kenji fly to Beijing by private plane to bring back Big Macs for the mini tyrant.

Mr Kenji reported that Jong-un was immensely spoiled and would rarely be told "no" or contradicted.

Even by the time Jong-un was born, his family had become massively wealthy as North Koreans starved.

Kim Jong-un's birth date is believed to have since been doctored to make it year later, (January 8) 1983, to mark 70 years since his grandfather's birth and 40 after his father's.

At school he was known as Un-pak, purportedly the son of a diplomat posted to Bern's Korean embassy, and was two years older than his class.

A former classmate told Frontline that Jong-un materialised in class, taller and bigger than everyone else, and struggling even to speak German.

"Nobody knew why he was there, he just kind of appeared," the anonymous classmate recalled.

Possibly because of his height, he relied on basketball to appear popular and successful.

Kim Jong-il would say that out of his children, Jong-un was the one who was most like him.

Jong-il's passion was golf, and he boasted to have landed 11 holes-in-one during his very first round of the game in 1994.

The claim would make Jong-il, who died in 2011, the greatest golfer ever to have played the game.

The US intelligence dossier on Jong-un's formative years says that at the Swiss school, he would sometimes declare to classmates that "some day you will all remember me".

Another distancing factor was that Jong-un lived in a luxury apartment and was collected by staff driving Mercedes.

He occasionally had other boys over to visit, and try out the "strange' (Korean) food the family ate.

When it came to Swiss food, Jong-un was gluttonous about Swiss cheese and took his enduring passion for the European cuisine back home to Pyongyang.

Jong-un was an 18-year-old with little academic record to boast of when he returned to North Korea in about 2000.

The following year, his half brother Kim Jong-nam fell from grace.

In February 2003, moves began to raise the profile of Kim Jong-chul ahead of his succession, but in time they fell away along with his father's respect as he became an Eric Clapton-obsessed wannabe musician.

In 2009, Kim Jong-chul was officially passed over when his father appointed his younger brother head of the Workers' Party of Korea and a four-star general.

North Korea was in no doubt of its next leader — the bad-tempered spoiled brat who never took no for an answer.