Kim Jong Un. ABC News photo

Seoul - As the world watches and wonders whether six decades of Kim family deification in North Korea can continue under new leader Kim Jong Un, speculation is mounting about who the son really is as details of his secretive past are patched together.

Little is known about the young Kim, the heir apparent of a nation possessing nuclear power and the world's fourth-largest military.  His name, age, marital status, and educational background remain sketchy and no one from the outside world has even heard his voice.

"His personality is known to be cruel and brutal. When firing the old officials, he does it very easily, very quickly," said Ha Taekeung, president of Open Radio for North Korea, an interest group based in Seoul.  Young but charismatic, Kim likes to speak more than listen at meetings, he said.

Kim is fond of China politically, but not culturally because of their low living standards. "It's because he is more prone to westernized lifestyle" from being raised "in a luxurious environment in North Korea and in Switzerland," Ha said.

Kim Jong Un's birth year was suddenly changed from 1983 to 1982 last year, making him either 27 or 28.  From 1998 to 2001, when he reportedly attended a school in Switzerland, he was known as Pak Un. But his real name had turned out to be Kim Jung Oon, according to South Korean intelligence, with the last character of his first name Oon meaning "cloud."  It was later changed to Kim Jong Un, with a different character that means "lighting up." Analysts have said this was a calculated move to justify the succession so as to make it appear that he was destined to "lighten up'" his father and grandfather's legacy instead of "clouding" it.

The world got a first glimpse of Kim Jong Un when his photo was published in state media Sept. 30, 2010. Many South Koreans were stunned because of his striking resemblance to North Korea's late-founder Kim ll Sung. Same chubby face with a slick pulled-back hairdo; he was even wearing the same china-collared outfit that his revered grandfather used to sport. That resemblance is exactly why he was "daddy's favorite" from early years, according to Kenji Fujimoto, who wrote a best-selling memoir about the Kim family after serving as their chef for 13 years.

Vienna resident Joao Micaelo told Reuters that he was a good friend of Kim Jong Un as a teenager, though he went by a different name then and his friends had no idea he was related to the North Korean leader.

"He was a big fan of the Chicago Bulls … His life was basketball at this time," Micaelo said. "I think 80 percent of our time we were playing basketball."  They would watch Jackie Chan action movies, do homework and play video games. "He was a good friend. He was very quiet. He was a nice guy."

Another former classmate at the Steinhoelzli school in the Berne suburb of Liebefeld told Reuters, "He was funny. Always good for a laugh."

South Korean media has been writing about Kim Jong Un with what little they can gather. Chosun Ilbo, one of the largest newspapers, chose five keywords regarding Kim Jong Un:  swiss-educated, competitive, military academy, rookie complex, and Kim Il Sung impersonation.

Referred to as a "young general" and "new star general" in his early twenties, the young Kim was hailed a few years later by state media as "a gifted person of military strategy and unrivalled genius" and a "genius who is well-acquainted in modern military science and technology."

This week, he became the "great successor" immediately after the announcement of his father Kim Jong Il's death. A day later, he was elevated to a "great person born of heaven," a propaganda term previously reserved only for his father and grandfather.

A song dedicated to the "young star" was taught in every village and by last year, and the state media officially started to refer him as general.  Since then, town propaganda departments were reportedly educating villagers on how the young Kim started shooting guns at the age of 3 and hit every target by the age of 9. According to Chosun Ilbo, they have also depicted him "driving a car at the age of 8? and "a basketball player who plays better than a famous professional."

North Korean state media reported earlier this month that Kim Jong Un's slicked-back, high-sided haircut is a fashion hit in Pyongyang and young men are queuing up at barber shops. Dubbed the "young and ambitious" look, state media quoted a resident Kim Moon Hyuk as saying "the flattop form looks full of spirit and passionate but at the same time gives a neat and elegant look."

A North Korean defector, Kim Hae-Sook, 50, testified to the Royal United Services Institute (RUSI) in London earlier this year that she heard from a high-level official in North Korea that Kim Jong Un underwent six plastic surgeries to look like his grandfather with chubby cheeks and a double chin.  Fujimoto had also told Japanese reporters that he did not recognize Kim at first when North Korea published the first picture in 2010, as he looked so different from his teenage years.

Meanwhile, South Korean activists and defectors are launching giant balloons containing propaganda leaflets into North Korea, according to media reports.  The leaflets, which oppose another hereditary power transfer in the North, have previously drawn the ire of leaders in Pyongyang. Some of the leaflets showed graphic pictures of former Libyan strongman Moammar Gaddafi's battered corpse and described his gruesome death.

It's unclear whether the body of Kim Jong Il will be displayed in a glass coffin next to that of his father, Kim Il Sung, who died in 1994.

North Korea has declared an 11-day period of mourning and a state funeral and a national memorial service for on Dec. 28-29.

ABC News' Seoyoung Cho and Sooyun Yum contributed to this report.