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 world 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.

Usage
Familiars don't seem to have any inherent uses to a vampire aside from substituting their own souls in place of the Vampire's soul when that vampire is fatally wounded, thereby allowing the vampire to come back to life. A particularly dangerous Familiar, such as another Demon or a well-armed human, could be used as a weapon against opponents. As seen with Alucard, a Vampire could amass a literal army within itself if it absorbed enough souls and likely become a transcendent vampire or something similar in the making, as research and a fair bit of "tampering" from the Hellsing family was involved in Alucard's creation.