The Bounts (バウント, Baunto) are a group of Humans that live by consuming Human souls (Pluses), plus Shinigami and Quincy souls which potentially increases their abilities.
Bounts are living beings just as Quincy are, but the difference being they have a special component that is encompassed by a Human soul. Bounts absorb Human souls, making them their own. It is these abilities that have named Bounts at various times in Human history as "vampires". As the accidental creations of a Shinigami scientist, they were once affiliated with Soul Society. Currently the Bounts in general are only a small tribe of beings from what they once were. Due to their circumstance they are neither affiliated with Soul Society nor the living world, they are pariahs, accepted in neither dimension.
The natural abilities they possess due to their unique creation differentiate them from normal Humans and even Qunicy. These abilities include:
Soul Absorption: Bounts normally try to absorb the Human's soul after the person dies and is trying to pass on. But they are also capable of taking the souls of those who are living, to do so kills the Human.
Immortality: By absorbing Human souls, a Bount can live forever. When they absorb Human souls, they gain eternal life.
Empowerment: By taking the souls of living Humans, Bounts gain unique abilities.
Powers & Abilities
The Bounts' inability to age is a product of how they survive: consuming Human, Shinigami, Arrancar, Hollows and Quincy souls which potentially increases their abilities. This also seems to make them infertile, as only one Bount, Yoshino Sōma, had the ability to reproduce out of the entire species. Normally, Bounts consume human souls that have already died.
Dolls
Bounts, being created as variations on a Shinigami soul, have similar powers to Shinigami. The foremost of these powers is the Bount doll, a familiar equivalent to the Shinigami's Zanpakutō. Like a Zanpakutō, a doll is its creator's power given physical form. Also like a Zanpakutō, a doll's personality is much like its creator's, though the dolls seem to have more personality variation than the Zanpakutō spirits seen thus far.
The main difference between a doll and a Zanpakutō is how they operate. Unlike a Zanpakutō, whose personality is generally dormant except when the Shinigami wishes to speak to them, a doll's personality is always dominant. Dolls are self-sufficient when released, though they can easily be resealed. The Bounts' ability to control their doll is largely based on their individual power and relationship with the doll. Moreover, if either the doll or its Bount partner sustains severe damage in a battle, the Bount may lose control over his doll, depending on the amount of damage dealt. If a doll is injured, it will simply return itself to its sealed state. However, should its master be injured, the doll may go berserk and kill its master. A doll may also kill a Bount who is not powerful enough to contain it. Bounts with a genuine bond with their dolls can transgress this (but only if their interaction goes well beyond that of between master and slave), however, as shown by Yoshino's doll's loyalty towards his master to the point where he refused to leave her side to fulfill her final wish: for her to be merged with him.
Like a Zanpakutō, dolls are generally sealed into an object and released as necessary. The object varies greatly from person to person and is often an expression of the doll's abilities. In order to call upon a Bount's doll, the phrase "Zeige Dich" is said, which is a German phrase meaning "Show yourself."
Bitto
Bitto are mosquito-like beings which drain the souls from humans and condense the energy into a purified liquid for the Bounts to drink. They even produce a useful vial in their sacs to store the elixir (which takes the form of a red, blood-like liquid). After the soul elixir has been ingested, the Bount who took it immediately experiences the effects, with veins popping from their skin.