User blog comment:B214/High School DxD New Episode 3 Discussion/@comment-16075832-20130722160729/@comment-16075832-20130723035539

I'm guessing you're an anime only watcher, the 3 factions are currently enemies which is why issei is sneaking behind Rias' back to assist them. Xenovia simply wishes to increase their chances of survival (Irina is fully prepared to die in the name of God but Xenovia would rather fulfil the mission and live). They simply have a common enemy right now.