Those accustomed to thinking in terms of US immigration law are in for a rude surprise.

Japan does NOT grant citizenship to foreigners. Maybe if you saved the Empress's life. But probably not even then.

No, marrying a Japanese citizen will NOT get you dual citizenship.

Japan is a tiny, highly urbanized, very crowded country. They don't like immigrants. They don't want your maladjusted weeaboo asses.