A familiar occurs when a vampire completely drains a non-virgin victim of blood, thus taking their soul and leaving a ghoul behind. Their minds and memories are assimilated into the vampire's, and can alter their personalities to an extent, as seen with Seras' newfound conidence after eating Pip There does not 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. Absorbing a significant amount of souls and accumulating an large amount of familiars may also be related to becoming a 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, as was the case with Luke Valentine. Schrodinger was asborbed by Alucard making him Alucard's only familar in the final chapter, but it is unknown how or if Alucard can summon him.
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 and regenerate, familiars can also manifest themselves in a corporeal form to assist their master during combat. 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, although research and a fair bit of "tampering" from the Hellsing family was involved in Alucard's creation.