Interesting, but why would Irys be evil if he was a guardian? This would also mean that he and the Gyaos are enemies, which isn't likely since he hates Gamera for killing his "family."
...I just re-watched the scene with Ayana talking to her cousin about all this, and the sub refers to "an enemy from the South" while the dub refers to "an enemy of the South," which leads me to believe that the sub translation could be a little off. This makes a little bit more sense, and Gamera, the Guardian of the North, would be Irys's natural enemy. I wonder if there is any signifigance in the Ryseicho and the Hydra constellation? Notice that the picture of the tortoise guardian shows it battling a snake.
Apparently Irys was created to keep humanity in check. I suppose the Atlanteans who thought like Kurata created both the Gyaos and Irys, while the more conservative-minded people of Atlantis created Gamera to "smash that kind of arrogance," as Nagamine puts it. Why the need to create Irys if the Gyaos were already doing their job? Plus, where did Irys's egg come from? Another Irys, or a Gyaos?

