Diplomats and escaped dissidents talk of a vain, paranoid, cognac-guzzling hypochondriac.
He is said to wear platform shoes and favour a bouffant hairstyle in order to appear taller than his 5 feet 3 inches.
But analysts are undecided whether his eccentricities mask the cunning mind of a master manipulator or betray an irrational madman.
"I know I'm an object of criticism in the world, but if I am being talked about, I must be doing the right things"
The analysis of him as a mercurial fantasist is certainly beguiling.
He is said to have a library of 20,000 Hollywood movies and to have even written a book on the cinema. He even went so far as to engineer the kidnapping, in 1978, of a South Korean film director and his girlfriend.
This taste for the exotic apparently extends to gastronomy.
Konstantin Pulikovsky, a Russian emissary who travelled with Mr Kim by train across Russia in reported that the North Korean leader had live lobsters air-lifted to the train every day which he ate with silver chopsticks.
The two men shared champagne with a bevy of female companions of "utmost beauty and intelligence", according to Mr Pulikovsky.
Mr Kim also has a reputation as a drinker. He was seen draining 10 glasses of wine during his 2000 summit with the South Korean President Kim Dae-jung and is known to have a taste for Hennessy VSOP cognac.
"I found him very much on top of his brief "
Former US Secretary of State, Madeleine Albright
As head of North Korea's special forces for much of the 70s and 80s, he has been linked by defectors to international terrorist activities, including the 1986 bombing of a Korean Airlines jet in which 115 people died.
Nor should it be assumed that eccentricity means inability. Mr Kim is said to assiduously follow international events on the internet, and some see him as a clever manipulator, willing to take great risks to underpin his regime.
Former US Secretary of State Madeleine Albright, who has met Mr Kim, said that the North Korean leader was very well informed and "was not delusional".
"I found him very much on top of his brief," she said, although she noted that some of the comments he made about his plans for the North Korean economy sounded illogical.
The cult surrounding Kim Jong-il extends even to his birth. He was born in Siberia in 1941 when his father, Kim Il-sung, was in exile in the former Soviet Union.
But according to official North Korean accounts, he was born in a log cabin at his father's guerrilla base on North Korea's highest mountain, Mt Paektu, in February 1942.
The event was reportedly marked by a double rainbow, and a bright star in the sky.
The younger Kim graduated from Kim Il-sung University in 1964, and after a period of grooming for leadership, he was officially designated successor to his father in 1980.
But he did not hold any positions of real power until 1991, when he took control of the armed forces - despite his lack of military experience.
Analysts believe he was given the position to counter potential resistance to his eventual succession.
After the death of Kim Il-sung in 1994, it was three years before he took over the leadership of the ruling Korean Workers' Party.