There are only a few Japanese Billionaires. Forbes has a list of 48 japanese billionaires and two almost billionaires. There are only about fifteen people with a billion dollars or more with a suitable age. Although the list could be expanded because of people with private wealth or they could be a close relative of one of the wealthiest people. You could have a 40-55 year old son of someone who is 75-95 on the wealthiest list. Most of the space tourists who have gone to orbit have been men. I think Anousheh Ansari was the only female space tourist.
Charles Simonyi and Dennis Tito both flew to orbit when they were 61 years old.
The most likely candidates are those younger than 60 or an exceptionally healthy person up to 70 years old. The person should not have a role where if they went to space their business interests would be at risk. They should not have large shareholders or a board who would veto a trip. Although they could retire or step down between now and the trip. The flight will not happen for four to six years. This rules even more of the older billionaires.