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They're referred to as Zainichi.

After WW2, there was over 1 million Koreans living in Japan. Some went back, some decided to wait due to the war starting.

Then the country they were going back to no longer existed, so they stayed. Many don't even have citizenship.

Many became yakuza because it's one of the few groups they could join. Hisayuki Machii (Jeong Geon-Yeong) was a legendary Korean yakuza leader

Yup I've heard the same, Yakuza leadership (also rank and file) has a lot of Zainichi.

I also remember hearing about how Pachinko parlours are usually owned by Yakuza/Zainichi and they funnel profits into North Korea.