He was also the step-brother to Kim Jong Un, the current supreme leader, whose mother Song Hye Rim was a local actress.
Before his brother became the premier, Jong Nam, being the eldest was named as heir apparent to his father and the next in line for North Korea's rule.
Both siblings share the same rotund facial features and similar expression but the two sons of Kim Jong Il have never met.
READ: North Korean leader's half-brother murdered in Malaysia - source
Citing reports by CNN in January 2012, Tokyo Shimbun journalist Yoji Gomi who was the author of "My Father, Kim Jong Il, and Me," said Jong Nam was very smart and knowledgeable about the world -- and open-minded, especially about economic reforms.
"My Father, Kim Jong Il, and Me," was published in January, 2012 - AP Photo
Gomi and Jong Nam have exchanged 150 emails in the past few years before the book was published.
He said Jong Nam voiced out against his father's 'military first' policy.
Jong Nam had also been previously reported to have attempted to use a fake passport to secretly visit Tokyo Disneyland in May 2001.
The incident was said to have changed his relationship with his father, resulting in his exile to China and later Jong Un's appointment.
Family potrait of Kim Jong Il and his son Kim Jong Nam circa 1981 - Reuters Photo
However, through his interviews with Gomi, Jong Nam strongly denied that his actions had caused him to fall out of favour with his father.
Jong Nam claimed that his name was dropped as a potential heir as he wants North Korea to embrace economic reform and open its doors.
Since then, he and his father has grown apart as Jong Il claimed that Jong Nam has turned into a capitalist.
In Beijing, during 2004, Yoji Gomi, a reporter for the Tokyo Shimbun, first met Kim Jong Nam.- AP Photo
In an e-mail to Gomi, Jong Nam said:
“After I went back to North Korea after studying in Switzerland, my relationship with my father had grown apart due to my insisting on reforms and open markets, and it was viewed with suspicion.
“My father felt very lonely after sending me to study overseas. Then my half brothers Jong-chol and Jong-un and half sister Yo-jong were born and his adoration was moved on to them.
"When he felt that I’d turn into a capitalist after living abroad for years, he shortened the overseas education of my brothers and sister.”
Jong Nam also said in the interview with Gomi, that he believed his youngest brother will fail as the "supreme leader".
Kim Jong Nam says the half-brothers were kept apart, following an ancient history of raising potential successors separately. - Reuters Photo
According to Gomi, Jong Nam was uncomfortable with Jong Un's appointment, as he sees the latter as still young, inexperienced and not adequately trained.
With those reasons, he felt that Jong Un's leadership is doomed to fail.
Jong Nam had been living incognito with his family in exile in Macau.
He was said to have two wives, at least one mistress, and few children in Macau and Beijing.
Jong Nam was seen frequently visiting Singapore and believed to have a lover living there. He was also known to have travelled to Austria and France for medical reasons.
Jong Nam in Malaysia
However, the reason behind his visit to Malaysia was unknown.
News of Jong Nam's murder was first reported by Yonhap news agency including a number of South Korean agencies, quoting unnamed sources. But no further information was given.
In the incident in Malaysia, police received details about the North Korean man who was trying to seek medical help at the customer service counter at the Kuala Lumpur International Airport 2 (klia2) on Tuesday.
The man was using a travel document bearing the name 'Kim Chol', born on June 10, 1970. He was reported to be at klia2 at 8am to board a flight to Macau at 9am.
Kim Chol was reported to be at the KLIA2 at 8 am to board a flight to Macau at 9 am.
Reports said he had died while on the way to hospital from klia2 after complaining of ill health.
