It could be any one from (From BBC)
The three remaining Kims
1. The first is
Kim Yo-jong, Kim Jong-un's younger sister.
2. The second is
Kim Jong-chul. He is Kim Jong-un's older brother, but has never appeared interested in politics or power. (He is known to be interested in Eric Clapton.)
3. Third and final one is
Kim Pyong-il, Kim Jong-il's half-brother. His mother - Kim Jong-il's stepmother - was angling to have him become Kim Il-sung's successor. She failed and was sidelined by Kim Jong-il as he rose in influence. Kim Pyong-il was sent to Europe in 1979, where he has held various ambassadorships, returning to North Korea only last year.
Others in Race are
Choe Ryong-hae
Chief of the politburo and is also first vice chairman of the State Affairs Commission. Last year he became the first new president in 20 years, replacing the aging Kim Yong-nam. He is the person who represents the North at international engagements.
Kim Yong-chol
He has been head of the United Front Department (responsible for relations with South Korea) and the Reconnaissance General Bureau, the country's main intelligence service.
other than these who could be in race are
Jong Kyong-taek is responsible for the State Security Department, which investigates and punishes political crimes.
Hwang Pyong-so is another official who has held top military posts and has run the OGD in the Kim Jong-un era.
Generals of the Korean People's Army (KPA)
Kim Su-gil and Kim Won-hong
Source :- https://www.bbc.com/news/world-asia-52450744