The Japanese don't come into the The True Korea timeline until the Sino-Korean War, when they decide to aid their former territory of Korea to help defeat the Chinese. After this war, the two countries end up becoming major allies. Thanks to assistance from the United States and Korea, Japan ends up becoming a major world superpower. After the fall of the Sino-Soviet Republics, Japan, along with Korea, takes their place in the council of five, cementing Japan's place as a world superpower. After the fall of the Sino-Soviet Union, the Japanese also annexed the Chinese island of Hainan.
Japan has an extremely powerful economy. Like in our timeline, Japan has the highest life expectancy of any country, and is also a major tech giant. With powerful allies like the United States and Korea, Japan is also a formidable opponent when in war.