@andre:
Big theory is that the fourth sibling is Julius. I'm not sure that makes much sense, but fans much more into the series than me seem to be into the idea.
Would never have conceived it myself, partly because I'm not really that invested. But, it was said the hell is ruled by 3 devils, controlling space, gravity and time. And the only time magic user we know of… Plus he's insanely strong, maybe there's a good reason why.
And he basically convinced both Vangeance and Yami to become Magic Knights. Maybe to guide them into cultivating their ability and become what he needed them to become?
Bigger stretches were certainly made in the history of fiction.
As a bonus, Magic Emperor seat would become empty.
It would suck, though. Maybe Tabata even trolls us with it till the last second.