One of the things I liked about Dawn of Sorrow was Celia's views and how, ultimately, they are subverted.
Celia believed that for God to be good, he must be opposed by a being of perfect evil, but Dawn of Sorrow spends much of the game proving her wrong, mostly by arguing that such a perfectly being doesn't and shouldn't exist at all. The closest thing we come to seeing a truly perfectly evil villain in the series is probably Menace in the same game, which is fitting, as its an artificial distilling of every single monster in (the fake) Dracula's Castle at that time into a single form.
There is no Satan or Lucifer in Castlevania because the only force that would go so far as to create such a being are humans. With very specific (potential) exceptions for the crap pulled by Death in canon (and it should be said we don't really know all that much about his original motivations or purpose), every act of evil in Castlevania is perpetrated in some way by humans. Even Walter, the progenitor villain of the franchise, began as a human. And my example of a perfectly evil being, Menace, was the result of a strictly human endeavor.
The closest thing in the series to a proper "Devil figure" would be Chaos, which on top of being a total plot cop out is left very much unexplained. We know Chaos can influence people very strongly, and we know that the Castle is something of an Earthly manifestation of it, but unlike in Lords of Shadow 2's Inner Dracula, we don't really know if Chaos is even intelligent or sapient, or indeed whether or not it is serving a purpose intended for it by God himself. It may easily by that Chaos is doing exactly what God means it to do, essentially helping to bring the "impurities" on man to the surface to be worked out, like silver.
But, in light of the answers we concretely have and the vagueries of everything else, I conclude that the universe doesn't need a special opposing force to God. It already has us, and according to Castlevania, we do a fine enough job of it ourselves.