Christianity took real hold in Korea through the effort of Protestant missionaries, mainly Presbyterian and Methodist. Catholicism had very little to do with it, and represents a very small minority of the Christian population of Korea.
It is notable that Pyongyang was once called the Jerusalem of Asia, because Christianity was the most active religion there. That was before the Russian-backed communists came in. Most Christians were killed, suppressed, or fled to the South. The remaining Christian churches in the North are fakes: showpieces by the regime, fooling only a few gullible foreign visitors ( including, sorry to say, Billy Graham and his family).
Putin’s Russia remains friendly with North Korea. So much for the Christian presences of the Putin dictatorship!
There were several systematic persecutions (including martyrdom) of Catholics in Korea a century before the first Presbyterian missionaries arrived on the peninsula.
10.9% Catholic vs. 18.3% Protestant means that the Catholics outnumber even the largest Protestant sects, which would hardly make them a “very small minority” of the Christian population in Korea.
Catholicism also has a very interesting history in Korea, in that the first Catholics in the 18th Century became Catholics not through the efforts of Western missionaries, but through a group of scholars reading, beginning in either 1777 or 1779 (the records vary), a number of written works that had come to Korea many years earlier, through China. These scholars set up their own Church, as best they could (Korea was all but sealed off from the outside world at the time),and until in 1784 one of their number( Yi Seung-Hun) was able to join the annual delegation to China, and made contact with a Jesuit there. He received further instruction (and Baptism—receiving the name peter!!!—the books didn’t cover everything) and then returned to Korea to baptize and instruct, which he did until he was beheaded in 1801 with more than 300 others.
These Catholics then worked various angles to join up with the annual delegation so that they might be confirmed and attend Mass, and the first foreign priest didn’t arrive until 1836 (It was a tough mission ground, as the general penalty was death, and the European priests kind of stood out—and even the first Korean priest, St. Andrew Kim, only managed to minister for two years before being beheaded in 1846.)