Not sure about this but i think the reason sera stick close by eu has got something to do about "reviving the vampire King (queen?") thingy and something about a prediction that spell doom for their kind.
and both Sera's and Yuki's faction do whatever it takes to prevent that from happening albeit doing it with diffeent methods from their side.I think yuki's faction opting to a more riskier path.
ayumu's sexy ass=nikuman??
I can see now if ayumu is making love with saras, the first place saras will bite out of love is his ass