Ryo, Kazuma, and Akira are all the same guy, but in parallel universes!
Not only the same blood type and same birth year, but all have their hair spiked up. Both Ryo and Kazuma have animals on their back. (Ryo has a tiger on his jacket, and Kazuma has a dragon tattoo.)
Shenmue started out as a Virtua Fighter RPG, with Akira instead of Ryo.
Ryo's father was murdered. Kazuma's parents were murdered.
Spaking of which, Ryo's dad's Japanese voice is played by Segata Sanshiro. Which basically means that Ryo is Segata Sanshiro's son(!) in an alternate universe. It explains why Ryo is such a good fighter, and why Ryo has a Saturn in his living room! (Segata is possibly Akira's grandfather in the Virtua Fighter universe)
So basically Ryo = Kazuma = Akira. And Segata is like the precursor or something. :lol:
I'm not into the Yakuza series, so maybe you guys could find better similarities!