Let me take you waaaaaay back. There was a time when games were developed on the fly, when quarters were fought over with rosters, when palette swaps were acceptable.
I can't imagine how hard gamers would complain if a fighting game featured a palette swap character, today. We've become a culture of whiners, you know. The times have changed, but that doesn't mean that those copy cat characters don't get their share of fandom. That's why Evil Ryu's arrival in the arcade version of Super Street Fighter IV is something to celebrate.
Evil Ryu will officially become available in Japanese arcades today. That other "secret character," Oni-Akuma will be available a little later on. Famitsu writes:
Each skill on Evil Ryu launches more slowly than regular Ryu, but tends to have more force. The control feel is nearly the same as Ryu, but the basic attacks that get used the most, like crouching medium or heavy kicks, launch more slowly as well. On the other hand, the opening after firing a hadouken is smaller, which seemed to make him pretty strong in projectile combat. Like Akuma, he doesn't have much energy, so fighting with him using the same style as Ryu will get you defeated pretty quickly.