Familiar

Familiars are Souls absorbed by Vampires.

Creation
A familiar occurs when a Vampire completely drains a victim of blood, thus taking their soul and leaving a Ghoul behind. There doesn't seem to be a limit on what a familiar can be, as at some point Alucard fought, defeated, and absorbed the soul of the hellish Baskerville Hound. Simple humans, such as Pip Bernadotte, can be familiars and can manifest into the real word as their host vampire allows. Draining the blood of another creature and turning them into a familiar appears to be, or be related to, going from a simple vampire to a True Vampire. Ingesting a significant amount of souls, thus accumulating a significant amount of familiars, may also be related to becoming a Transcendent, nearly unkillable vampire such as Alucard. Alucard could also devour beings via Baskerville, but they didn't seem to be at his disposal when he did so and killing the dog could release the person(s) back into the corporeal world.