Luzi pretty much covered it, but here is my take/experience.
We got involved with a startup bank a bunch of years ago. We were a customer, but learned a lot of the other sides.
3 guys with banking experience got together. One had been a bank president, one a commercial lender specialist and the third was in bank operations. They wanted to create a bank focused on smaller businesses and entrepreneurs. Specifically helping them with SBA (Small Business Association, in the US, established to give government backed loans....its hell to work through, but great if you can get one).
Enter wife and I. We had nothing. They knew that. That had basically nothing., but they were rounding up investors. Basically, 'Hey, deposit a bunch of your money here and we will pay you interest on it'. Then they would take that money and loan it to poors like me and charge more interest on it. The difference in interest was their profit. If you don't have enough investors, and don't give enough loans, banks fail.
The key to their success was the SBA aspect. No normal, established bank would talk to me, but this new bank, dealing with SBA, they helped us. Then 10 years later helped us again. In that 10 years they became the biggest commercial lender in the state. When doing these commercial loans, they also increased their personal banking because all of us commercial customers moved all of our personal lines over. Now we have other banks trying to get us to move our loans and money there. Quite funny.
I have no idea how any of this would work in Japan.....with the whole literal 'Fall on your sword' thing going on. But the key here is to find a niche that often involves more risk and....having the right people in place. For you, if you were willing to take the risk, would be to be an investor and recruiting the right talent.