Yakuza 6: Song Of Life

There is a hidden safe in Chapter 1 that requires a passcode to open it, here's how to get your hands on what's inside.

 As soon as you arrive in Kamurocho in Chapter 1, head to the New Serena rooftop on Tenkaichi Street by heading down the alley next to the Golden Triangle bar and up the stairs until you reach the rooftop. The safe is in the middle of the rooftop area, and the code is divided in two parts; one in Chapter 9 and  the other in Chapter 10 of the Yakuza comic series. The complete code is "KP56VFA9MX2B7LH", and inside you'll find a Rainbow Bun and 1 million yen.