Building credit for Japanese wife

I want to start building credit in Japan for my Japanese wife so that she will have access to credit (i.e. bank loan, credit card, etc.) if we ever decide to move back to Japan such as when we retire. She has lived outside of Japan for more than 25 years but still has family in Japan whom she visits every year. And she is still on her family's koseki. Is it possible for her to somehow build credit in Japan while still living and working abroad? If so, what are some of the best ways of doing this? Buying a car, opening a bank account, obtaining a credit card? Would one of her family members or parents need to co-sign on a loan or credit card and would that still help her in building her credit? I would be very grateful to anyone who has experience or knowledge to share on this subject.

Japanese credit history is not something you "build". It's a kind of deduction scheme.
Credit card companies evaluate her when she applies for a credit card/loan. They check your job, how long she has worked for the company, her income, if she has other loans etc based on their own standards.
If she is on a blacklist which credit card companies and banks share, she would fail. If not and she matches their standard, she would be accepted.
They will do "fact check" by calling her Japanese company to check if she actually works for the company. Many of credit card companies and banks require applicants' companies have a Japanese phone number through which they can communicate in Japanese.

The fact check is done like this.
"My name is Tanaka (random Japanese individual name, not company name). Can I talk to Ms. XX(the applicant's name) who belongs to your team?" If the person who picked up says "there is no XX here", she would fail.
So, the Japanese phone number can't be HQ number.

Buying a car, opening a bank account, obtaining a credit card?

Any legal resident can open a bank account in Japan, so she can do that once she legally starts living in Japan. There are many affordable cars that are easier to drive for women, so it's wiser to stock cash enough to buy one rather than to "build credit".

Actually, I wonder if she already has a bank account in Japan under her own name, because Japanese nationals who frequently visit the country often do.

As for credit cards, it can be easily made for a resident with a stable income. But if she doesn't have any income, and if the head of her household doesn't either (which is not uncommon for aging housewives with retired husbands), I don't know how easy or hard it is to make one. Perhaps someone can give advice on that.

But Japan is not really a country where you gain credit through credit cards. Cash (or the remaining amount of your bank account) works best.

Thank you to those who replied to my questions about credit. I realize now that Japan is a more cash-based society than perhaps some other countries and that credit does not work the same way as in the US. It sounds like there are some of the same requirements of borrowers as in the US such as having a steady income and job, etc., as I expected. This information is all very useful to me.

I think that my main concern is her having the ability to to rent and/or buy an apartment/house in Japan if she or the both of us ever decide to return to Japan for some reason. Since she has not lived in Japan on a permanent basis for many years, she does not having any financial history in Japan which is a little worrisome.

As stated above – history is only meaningful if it is negative. When buying a house/condo for the first time it will be all about the down payment, appraisal to loan value, and source of funds to pay off the mortgage. If already retired, then a cash purchase makes sense – there being no tax value in having interest expenses.

Thanks for the advice. I will keep in mind.