Haha. Anons, you want to know the main mystery right from the start, eh? There’s no way Kagami will be so accommodating, he promised to reveal things little by little, and the Michaela novel series has only just begun (compare: 2 volumes against 6 volumes of the Guren novels).
So what little this novel gives on who Michaelas are is literally only a couple of sentences in chapter 4:
The organization known as the Hyakuya Sect that raised Mika, had to know what meaning the name “Michaela” was loaded with. A boy carrying the Michaela factor was born one in tens of millions. Finding such a child was incredibly difficult.
So we know for sure that only a male child can be a Michaela. In chapter 3 Ferid also devulges that he was targeting Crowley long before Crowley was even born. And this is all we know for now. When Ferid is killing the Hyakuya orphans, he’s waiting for Krul to arrive, but she’s not coming and Ferid is wondering if she’s OK with a “Michaela” being killed here when it’s practically impossible to find another Michaela in the destroyed world with the human population reduced so drastically.
The theory about the original Michaela being an angel (but not Archangel Michael) damned by God and becoming the first vampire and the first progenitor is a fairly obvious one, which is the precise reason why I have my doubts about Kagami actually going with it… besides, we need to tie in that Michaela factor somehow, and here your guess is as good as mine…