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At the end, it was concluded that Ruby might had been brought all the way where the Vampires come from within the continent, a big kingdom of Vampires hiding beneath snow mountains which is controlled by Vampires, but where most of the population are humans, which are the preferred blood of vampires. This also explains why various Vampires we have encountered were Humans, Rose being one, I think. Humans seem to be the favorite snack of the bloodsuckers, and they're probably rising them like cattle without these people even realizing it, it is both sad and depressing, but at the same time somehow understandable? I guess they need to get it from somewhere if they want to sustain themselves. Apparently monster or animal blood can keep them at bay, but they need human-like blood to keep themselves sane.
Check "The Protagnist's Sister Is Actually the Strongest"!