[Irenesharda]

My idea is that the Kamitsuki are a sort of specialized Magius, a subspecies sort of. Saki says that Marie was the first Kamitsuki and she's right. Marie was the first human that was transformed into a Magius, taken from one species and changed to another. Cain and all the Magius on Earth are naturally born aliens that have been around a long time and keep up appearances by taking the bodies of humans, who also happen to double as their food source.

So the Kamitsuki are humans who were turned into aliens using artificial, biochemical means. The others Magius were all born that way.