The local Korean deli uses cheese in many of their dishes; my friends from several other East Asian countries have commented that they still consider cheese to be unusual and not part of their normal diet. It is entirely possible that this is a sampling error, but I'm curious when cheese entered the Korean diet. (I did some google searching, but the results about modern Korean diet overwhelm the historical sources, and the historical sources don't refer to cheese at all.)
1959, by a Belgian missionary named Ji Junghwan, who helped started domestic Korean cheese production at Imsil County.
Cheese was introduced to South Korea in 1959 by a Belgian missionary, who came to Jeollabuk-do to help people surviving the Korean War.
Lee, Cecilia Hae-Jin. "Keolla Do" Frommer's South Korea. 2nd ed. Vol. 775. Hoboken, NJ: Wiley, 2008. 19.
It was out of youthful ardor that he chose to be a missionary in Korea right after the war ... Ordained a priest in 1958, he arrived a the Diocese of Jeonju in 1959 and soon took on quite a few big adventures ... when he was appointed to Imsil he raised mountain goats and started to make cheese.
- The Korea Foundation, "Korea Focus - September 2012".
This marked the appearance of cheese in the Korean diet. Cheese is not a component in conventional Korean cooking, however, and consumption thereof remained (and in relative terms, continues to be) light for decades. As late as 1990, total cheese consumption was only 7,000 tonnes - around 20 grams per person.
Korean appetite for cheese has since experienced tremendous growth, driven particularly by the adoption of western style food such as pizzas. Nonetheless, as late as 2013, the South Korean per capita consumption of Cheese was only 2.2kg. For comparison, the average American consumed 15.7kg of cheese in the same year.