Familiar

Familiars (使い魔,) are souls absorbed and bound into servitude by vampires.

Creation
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 confidence after eating Pip Bernadotte. 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.

Depending on whether they gave their blood willingly or had it taken by force, familiars show varying degrees of control over their own actions. Pip was still the same person as before and Alucard's Romanian soldiers appeared to be loyal to him out of free will and were still gracefully riding into battle. Most familiars seem to not have control over their actions and appear mostly mindless. 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 a large number of familiars may also be related to becoming a nearly unkillable vampire, such as Alucard. Alucard could also devour beings, like the hounds of 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. Schrödinger was absorbed by Alucard making him Alucard's only familiar in the final chapter. It also seems that only actual vampires have the ability to have a familiar; meaning that artificial vampires like the Valentine Brothers do not have this particular ability.

Usage
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 and some familiars such as Rip Van Winkle and Tubalcain Alhambra can be manifested with the weapons they used in life. 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 were involved in Alucard's creation.

List of notable Familiars

 * Tubalcain Alhambra
 * Rip Van Winkle
 * Schrödinger
 * Pip Bernadotte
 * Baskerville