Han Jae-Moon: the feuding fraternal states of the Korean peninsula found themselves with unprecedented opportunity in the second third of the 21st century. Their eternal unresolved civil war slowly petered out as new generations replaced older bitterer ones, and the great powers to the east and west that once sponsored their protracted struggle themselves fell prey to dissolution. By this time the autocratic north, though still keeping its ultrarrevolutionary pretensions, was ruled by a hereditary tyrant who was tired of seeing unending hostility with no end in sight. The democratic south, shorn of the foreign advisors who had cheered on the fight, was willing to find compromise. And so the most improbable terms of reunification was ratified by both sides: the north, with all of its industrial and military might painstakingly rebuilt over the last century, would allow free elections and transition towards multiparty democracy. The south, with its mature late-stage participatory republic, would accommodate the integration of their kin less used to its abundance of choice. So, the former Supreme Leader of the north would be made the ceremonial head of state of United Corea, transforming the new country into a constitutional monarchy presided by a Son of the People from a certain bloodline.
As preposterous as the arrangement was, after nearly a hundred years of conflict, both sides were willing to lay down its arms over a settlement that pleased no one. Nationalists on both sides were bought off, expelled, or otherwise take care of. Despite the elevation of their leader, hardliners from the North bristled at thought of him made into a puppet. Many departed to the west, to China, where they would ironically fight on both sides of the Golden Dynasty's struggle with Crimson Secession forces, another conflict involving both neo-monarchists and neo-Marxists.
The Kim in question was quite happy with the arrangement himself; he was tired of his regime's continued status as pariah, unending spending on weapons, and not being able to take his children to Busan Disneyland and MorganPark Seoul. The southern president was relieved to find that her counterpart had indeed made effort towards beating ICBMs into plowshares, and that the economic disparity between north and south wasn't as high as it was in prior generations. (Though that was also a dire sign of the accelerating impoverishing of the world.) The northern military and intelligence officials that did join the new union were very effective, and did have their uses.
United Corea moved into the void left open by their falling former allies. The south's economic and cultural vitality was bolstered by the north's survival-at-all-costs cohesion. Their national achievements included a space program built from both country's efforts, a new seaprogram that investigated solutions to the rising tides and experimented with both underwater and surface aquatic habitats while Tokyo flooded, and a new wave of dazzling entertainment exports from K-song to VR gaming to holo-dramas produced by southern expertise and northern 'promotional techniques' while Hollywood burned. This era of good feelings led to a Corean-led peacekeeping force in China during the interregnum after the Crimson Restoration. This diverse conflict included new citizen-soldiers naturalized from the foreign military bases that existed in the divided north and south. The newly-integrated Eighth Army, formerly of the U.S., performed well even after the Coreans had nationalized their base.
Even after all this, the Coreans recognized that their new nation was doomed, along with the rest of the planet. So the new government began a new initiative to preserve its revitalized civilization in the stars themselves. Codenamed Chungsu, "appendix", the intention was to ensure that their people, culture, and identity would survive Earth itself, even as an unobtrusive vestige within a living organism. A vestige with potentially great leverage over the body politic indeed. To achieve this, the government intensified its space and sea programs, cooperating both with the Russian Republic and Great China on the former and expanding upon the latter with the aim of building underwater cities in the Namhae shelf and the Ulleung Basin. One of its efforts was to recruit a future generation of able explorers and pioneers who could survive in these harsh non-terrestial environments. This was done with an intensive and invasive evaluation of all of the country's inhabitants and in the diaspora. Thus, Han Jae-Moon, Gangnam chaebol scion, found his way into the service of his country.
Notes: the Civilopedia refers to him as "Moon", but I'm pretty sure his surname should be Han, since Jae-Moon is hyphenated and likely a given name in the same way Sheng-Ji is.
Apparently the name of the sponsor is 청수, or Cheongsu, which means "blue water" or "crystal clear water". ( Which would be a fitting name for a Rising Tide faction, but lol Firaxis had to screw up the transliteration (as with the Moon vs. Han surname thing) so I'm going to stick to the appendix etymology I made up after Googling for "What does Chungsu mean in Korean?" Just really lazy editing, guys.