Kind of over simplified, Peralisc, but yeah.
They were made by "the gods" (I'm assuming Greek since they never specify, could be Atlantean for all we know) in opposite pairs (White girl/Black guy for example). They were supposed to watch over, protect, and guide humanity. Whenever a pair met, they would often marry and stay together. Being opposites, staying so close together caused both of their powers to fade away.
They seem more like demigods simply for the fact that when separate, they were powerful but together they were normal humans.