Kim Il-sung, born Kim Song-ju, fought with the Chinese Communists forces in eastern Manchuria
In his work “On eliminating dogmatism and formalism and establishing Juche in ideological work” (https://www.marxists.org/archive/kim-il-sung/1955/12/28.htm), Kim Il-sung mentions a few left-wing groups in Korea, such as the KAPF (Korean Artiste Proletarienne Federation), as well as the Tuesday Group and the ML (Marxist-Leninist) Group which were two communist factions established in Korea during the 1920s and 1930s. I make note of this to lend legitimacy to some of the facts written in this server, as I suspect there to be hesitation among many to accept that Korea had a rich communist history long before the establishment of the DPRK in the 1940s.
Currently my opinion on Kim Il-sung has not been cemented. He is not among the old communists who struggled to bring marxism to Japanese-occupied Korea, but rather part of a new group. His revolutionary activities reportedly begin in 1932, almost 15 years after underground movements began by both Korean communists and nationalists for the struggle of Korean liberation. On him, I need to read more. It is interesting to note that in the work linked above, published in 1955, Kim Il-sung talks about the failure of the DPRK propaganda workers to effectively teach the history of the Korean struggle