SK were like the Jews and Japan were like the Nazis but in Asia. This is oversimplification of it and it was a different kind of genocide.

Japan never really apologized or rather when they (politicians or leader figures) tried, it was often taken back or trivialized by other Japanese politicians or anonymous civilians making death threats to politicians who made attempts to apologize. There is still a strong alt right wing Za Warudo cult following in Japan that believes they did Korea and China a favor raping the women by the thousands and mutilating them because they brought industrialization.

While Japanese civilians are great and completely different from back then, politicians and the upper-echelon types are different. Remember when Ukraine compared Hirohito to Hitler and Mussolini? The comparison is pretty accurate and apt, speaking as a Korean myself who has Japanese relatives not by the war but by relatives marrying. Japan got angry at the comparison and demanded they apologize. The attitude of how Japan treats their atrocities of the past vs how Germany is very different. Let's be real; if Ukraine never apologized for that comparison of Hirohito = Hitler/Mussolini, Japanese government probably would have rescinded a lot of what they would later donate soon after the apology as aid in the war.

I think SK and Japan definitely can be allies and get to a better place. But the issue is everyone just wants to ignore these big elephants in the room and pretend the problems will solve itself. A lot of people will for example try to paint some of the resentment I feel as propaganda from China or CCP shilling which I have been called before. But until we address the elephants in the room and reconcile with these facts, we aren't going to get anywhere productive.