Simply complete a total of 40 substories with Kiryu and/or Majima.
You can get a Substory locator to help you find them as they will be marked with a ! on the map,
For Kiryu -
Finish the substory "The Tax Lady".
For Majima -
Max out your friendship metre with the chef at Komain.
However 40 should be easy enough to find, just talk to anyone you can, and they will appear!
A full list, however, can be found here -
https://gamefaqs.gamespot.com/pc/241144-yakuza-0/faqs/74451/...They are numbered, so using the menu you can help determine which you haven't done yet. Some only unlock after a certain chapter also.