Soul

The soul is the immaterial essence or life force of a creature. Within the Hellsing universe, the soul is carried within the blood and is thus considered the "currency" from which vampires derive both their longevity and the power to conjure familiars and regenerate their wounds.

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The currency of the soul is blood -- once blood is drained from a person, their soul is taken into whatever drained them and can be used as a familiar by Vampires. What is left behind is a ghoul, an undead agent completely submissive to the will of the vampire that bit it. Even Vampires and other non-human creatures, like hellhounds and horses have souls, though demon souls are considered to be "impure" and "damned". They too are subject to be drained and used as a familiar.