I lived in Japan and had a Sempai (mentor). You do lots of menial jobs for them for a very long time. This proves you have the tenacity required so they're not wasting their time and allows them to profit off you from getting cheap/free labour. Eventually, if you show self control, they may allow you to step up with learning. This can take 3-5 years. I don't think many people are willing to go through it, frankly. Sounds nice but it's a hell of a sacrifice and a gamble. Your mentor might be a dick.