As North Koreans face an uncertain future without Kim Jong Il, the world's attention now turns to his son, whom the ruling Workers' Party has dubbed the "great successor."

"Kim Jong Un's leadership provides a sure guarantee for creditably carrying to completion the revolutionary cause ... through generations," the letter said.

But little is known about the deceased leader's youngest son. Even his age is uncertain to most of the outside world: he is believed to be in his late 20s or early 30s.

He is believed to have been schooled in Switzerland and is thought to be capable of speaking some English and German, and possibly some French.

He is said to have a fondness for Michael Jordan and James Bond.

In October 2010, he was promoted to the rank of four-star general just before a rare meeting of the country's ruling party. The announcement was the first formal mention of his name in official state communications.

At one point, Kim Jong Il's eldest son, Kim Jung Nam, was considered the top candidate to succeed his father before he fell out of favor after he got caught trying tosneak a trip to Tokyo Disneyland using a forged passport.

In an interview with Japan's TV Asahi last year, he said he opposed the "hereditary succession for three generations."

He, however, added, "I would like my younger brother to do his best for the people of North Korea and their true wealth."

But Kim Jong Un is simply "not ready" to rule, said Victor Cha, a former White House National Security Council member and an expert on North Korea.