Ryu's most relevant in the original game for obvious reasons and the Alpha series since the whole Akuma thing is really important, or more so than other games in the series.
SFII is all about Guile and Chun-Li.
SFIII is about Alex.
SFIV is about Guile, Chun-Li, Viper, Abel, and Juri.
In all of those games, Ryu is just there then Akuma shows up and does his "dark side" schtick and they fight.
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I like how each new topic you make reveals such varied facets of your idiocy. - foolmo
Now this is entertainment!