You know, guys, the more I think about what the Michaela factor might be, the more lost I get. If we take Christianity in general, seraphs are only one type of angels (Christian_angelic_hierarchy), and there are so many possibilities as to what type the Michaela factor might represent. It all depends on how much Kagami knows about Christianity.
In the first place, how much the end of the world advanced 8 years ago after the first Owari no Seraph activated? It was stopped midway but at what point? The 7 seals go first; from what we see in the manga, it appears that all of them have been released already. Then go the 7 trumpets that are also underway, although they apparently aren’t released in proper order and basically what humans tamper with. And then there are 7 angels with 7 bowls. As a very far-fetched and unlikely wild idea, what if Michaelas represent those last angels. Argh, don’t listen to me, I’ve read up too much on the Apocalypse and am now more confused than ever.