Japan has very few regulations regarding lending to individuals. Each company has it's own contracts which are subject to wide intepretation. If you sign (or hanko) a contract *that* company is responsible for recovering the money not the law. They can and do change the rules if they think you are getting risky.
Some will say that private borrowing is unwise but if you *really* intent to pay the money back *and* you have a solid plan to do so *and* you can afford the high interest you can get a loan quite quickly and pretty easily so it will cover you until better times arrive. If you don't have any of the above you will soon find youself in very deep and *forget* all thoughts about splitting without paying up. It's been tried many times and they are up to all the tricks.
I've borrowed from individuals, banks and institutions and have never had much trouble because I always pay back according to the letter of the contract. Getting started as a borrower may be difficult unless you have some upstanding individual willing to co-sign.
Good luck.