It's almost impossible for a non-Japanese person to become a citizen of Japan, even if your family has been living there for three generations. Essentially, the only way to become a legal citizen is to marry a native, and even then, it's not a sure thing.

Then again, this is coming from the society that discriminates against ethnic Japanese whose ancestors worked "unclean" jobs. Even though those jobs aren't considered "unclean" anymore. So really, the Koreans are kind of screwed.