Japan does not have a fiancee visa. You are not able to obtain any type of visa based on your relationship with your Japanese girlfriend. A working holiday visa will be one of the good options to stay up to one year which will not be extended. When you are employed or start your own business in Japan, you would have a chance to apply for some type of work visa. Each work visa has its own requirements.
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How can I qualify for Japanese residency as a remote employee?
My girlfriend and I are looking into my Japanese visa options starting from October. After our 4-year relationship in Germany, she needed to move back home to Japan. I am a Finnish citizen and my Finnish employer is willing to let me work remotely from anywhere in the world. Since the working holiday agreement between Finland and Japan is still under processing, what are our best options as a couple? Under these circumstances, how can I obtain residency in Japan?