His Three Sons: North Korea's Shifting Heir Apparent

Dec. 19, 2011— -- With North Korea announcing the death of supreme leader Kim Jong Il, the country is urging its people to rally behind his youngest son and heir-apparent Kim Jong Un.

Although the age of Kim Jong Un is unknown, it is believed that he is 28 years old. His father revealed his third son would be the head of the Korean Worker's Party and de-facto head of state of North Korea in 2009, and had placed him in high-ranking posts. Kenji Fujimoto, Kim Jong Il's personal sushi chef, wrote in a memoir that Kim Jong Un was the favorite because of his good looks and mannerisms that are similar to those of his late grandfather, Kim Il Song.

Kim Jong Il inherited power and was named the dear leader after his father, North Korean founder Kim Il Sung, died in 1994.

"At the leadership of comrade Kim Jong Un, we have to change sadness to strength and courage and overcome today's difficulties," North Korea's official news agency said, adding that the people and the military "have pledged to uphold the leadership of comrade Kim Jong Un," a "great successor."

Kim Jong Un is the second son that the leader had with Ko Young-hee, a Japanese-born Korean and dancer who played the role of North Korea's first lady until her death in 2004. Although little is known about her death, it is believed to have been from cancer.

Kim Jong Un is known to be a fan of NBA basketball, active, and competitive -- but reports say that like his father, he has diabetes and high blood pressure.

Kim Jong Un, who was educated in Switzerland, accompanied his father on trips around the country over the past year in the private armored train he was known to exclusively travel in. The North Korean dictator died on board the train, according to state media, which called it a result of "great mental and physical strain" during a "high intensity field inspection." It is unknown if Kim Jong Un was with his father at the time he died.

There hasn't been a statement yet on the country's official succession, and it is still unclear what is occurring inside the Korean People's Army regime, or if high ranking senior officials are actually backing him up.

Kim Jong Un's uncle, Jang Sung Taek, who has much more political experience than his nephew, will likely be a powerful influence over the new regime if the young man takes power.

"I don't think he's a threat, but his advice to Kim Jong Un may carry heavy weight, particularly during the next few months," Donald Gregg, former U.S. ambassador to South Korea, told ABC News.

Jang, 65, has held powerful positions in the military and secret police. He is a member of the ruling Workers' Party and acted as assistant -- and many believe de facto leader -- to Kim Jong Il.

"[Jang] Song Taek is a politically significant figure of increasing prominence ... he has been entrusted by Kim Jong Il to 'guide the government' until the 'young emperor (Kim Jong Un) is ready to assume power,'" according to a U.S. diplomatic cable from the U.S. consulate in Shanghai to the State Department that was posted by Wikileaks.