I searched for Ryu's lastname and found that it is "Hoshi", but somehow I get the feeling this is only used in the "crappy" movie Street Fighter. Here are the sources I have found so far: Wikia, Wikipedia. The name is always listed under Action Movie or Live Action.
Is Hoshi his "real" name as used by Capcom in the official Street Fighter Series? Perhaps Ryu has no lastname and Hoshi is only used for the movie.