The leader of the SAC (formerly NDC) is the “leader” of the country, voted on by the SPA, which is elected by the people (SAC leader is also in WPK leadership as the CPC in China). This bias is attributed to the extreme conditions the dprk is under and the trust people have in the family.

Kim Jong Un is General Secretary of the Workers Party of Korea, and Chairman of the State Affairs Commission. These positions are elected by the WPK Party Congress and by the Supreme People’s Assembly respectively.

If Kim Jong Un didn’t wish to continue to hold his positions, one of the Vice-Chairpersons would take his place temporarily, and a successor would be discussed and elected at the next party conference, also likely a Vice-Chairperson.

For example, Kim Jong Il was elected into the Party Central Committee in the 70’s, and in 1974 was elected as the successor to Kim Il Sung. Jang Song-Thaek was elected to succeed Kim Jong Il, however, he wanted to reform certain areas, thus debate regarding his intentions and whether he was a revisionist or not ensued; the party then switched and had Kim Jong Un succeed Kim Jong Il.

Jang Song-Thaek then staged a coup in an attempt to consolidate power by force (confirming his intentions were not pure and that he was likely a revisionist in the intent of his “reform”). He was executed thereafter.

It’s important to mention Jang Song-Thaek to show that a successor to Chairman of the SAC doesn’t have to be a direct child of the former.

So, if Kim Jong Un were to retire, or wish to discontinue his positions, it would be somebody in the Politburo, or a Vice-Chairperson of the State Affairs Commission, to succeed him. However, there currently isn’t an elected successor appointed, because likely odds are that he isn’t retiring or dying in the near future.

All of Kim Jong Un’s roles within the DPRK are: Supreme Commander of the KPA, Chairman of the Central Military Commission of the WPK, General Secretary of the WPK, Chairman of the SAC, and Supreme Representative [as entailed in WPK leadership]

Kim Jong Un is not actually in total control of the DPRK; the Supreme People’s Assembly has, by far, the extreme majority of control over the latter. Kim Jong Un has never been in either the Supreme People’s Assembly or its respective Standing Committee. Premier is the second top rank within the SPA, currently held by Kim Jae Ryong [not related]. President of the Standing Committee (Presidium) is the top position within the SPA, a position held by Kim Yong-Nam [not related] until April 2019, where Choe Ryong-Hae thereafter was elected. That isn’t to say that Kim Jong Un holds no power within the DPRK, but anyone within the SPA certainly has more legislative authority.

Each person within the SPA, including Premier and Head of the Presidium, is elected (and thus their power is temporary and can be removed at any time). The closest thing to a dictatorship (so to speak) in the DPRK is the Dictatorship of the Proletariat, a determining class dictatorship of the majority which governs the state.

The only of the Kim family members (remember this is a common Korean surname, I am referring to the lineage of Kim Il Sung) to have an SPA position was Kim Il Sung, and he abolished his position. The “next in line” leader in the WPK is likely not to be a descendant of Kim Il Sung either.