Hollows are a central race of spiritual beings that serve as primary antagonists within the narrative. A Hollow is born from a human soul, known as a Plus, that does not cross over to the Soul Society after death. When a spirit lingers in the human world due to regret, despair, or a powerful attachment, its Chain of Fate, the link connecting it to its former life, begins to corrode. Once this chain is completely consumed, the soul undergoes a transformation called Encroachment, becoming a Hollow. This process creates two defining physical features: a hole somewhere in the body, representing the loss of their heart, and a bone-white mask that conceals their former identity.

The fundamental nature of a Hollow is defined by emptiness and an overwhelming, instinctual hunger. They are driven by a perpetual need to devour other souls, both of the living and the dead, in a futile attempt to fill the void left by their transformation. Most basic Hollows are purely feral, acting on animalistic impulse rather than malice. However, some retain their intelligence and cunning, making them significantly more dangerous. A notable exception is the rare Hollow that can express loyalty or even a degree of non-malevolent behavior, though these are not the norm.

Hollows primarily inhabit Hueco Mundo, a desolate desert dimension that exists between the human world and the Soul Society. They travel to the human world by tearing open the dimensional fabric using a technique called Garganta. In the story, they function as the primary enemies of the Soul Reapers, who perform a Soul Burial to purify them and send their cleansed souls to the Soul Society. Quincies, on the other hand, destroy Hollows entirely, erasing them from the cycle of reincarnation.

The evolutionary path of a Hollow is a brutal, cannibalistic hierarchy. To gain power, they devour one another. When a large number of ordinary Hollows consume each other, they fuse into a single massive entity known as a Menos Grande. These are further classified into three levels. The lowest, Gillian, are towering giants with little intelligence, acting as mindless foot soldiers. If a dominant personality within a Gillian consumes enough others, it evolves into an Adjuchas. Adjuchas are smaller, intelligent, and possess individuality, but they must continue to feed on other Hollows; if they stop, they will regress back into a mindless Gillian. The final, most powerful evolution is the Vasto Lorde. These are humanoid in size and shape, possess intellect surpassing that of a captain-class Soul Reaper, and are incredibly rare.

A Hollow that removes its mask gains Soul Reaper-like powers and becomes an Arrancar. This process, often facilitated by the Hogyoku, allows them to seal their Hollow abilities within a sword called a Zanpakuto. Arrancars are far more human in appearance, retaining only fragments of their mask. Their unique abilities include Hierro, or steel-hard skin, and Sonido, a high-speed movement technique. The most powerful Arrancar are ranked as the Espada.

Key relationships define the role of Hollows in the story. They are the natural prey and antithesis of Soul Reapers, who purify them, and Quincies, who destroy them. Specific named Hollows, like Grand Fisher who is responsible for the death of the protagonist's mother, or Acidwire, who is the brother of a main character, create personal conflicts that drive character development. Other Hollows, such as Nel Tu, become temporary allies. The Soul Reapers who underwent Hollowfication and gained Hollow masks are known as Vizards. The ultimate development for a Hollow is to break the cycle of hunger and instinct either by becoming a Vasto Lorde or by removing their mask to become an Arrancar, achieving a new, stable form of power and individuality.