Hollow are a race of creatures which are born from Human souls who, for various reasons, do not cross over to Soul Society after their death and stay in the Human World for too long. They are corrupt spirits with supernatural powers that devour the souls of both living and deceased Humans. Hollows are the principal antagonists at the beginning of the story and remain a primary threat beyond that point. Hollows settle in Hueco Mundo, but can cross over to the Human World and Soul Society.

Hollows are former Pluses (deceased human souls) that lose their hearts to despair or remain in the real world for too long. In the anime, non-human spirits can also transform into Hollows. Any spirit that is not guided to the Soul Society by a Shinigami via Soul Burial may eventually descend into a Hollow.

Transformation Process
The majority of ghosts peacefully move on to the Soul Society after their deaths. Some, however, stay in the living world to fulfill certain goals, such as looking after a loved one, protecting a particular location, or pursuing a grudge from beyond the grave. However, with only limited abilities to interact with the living, and by definition tied to their goals, many ghosts find they are unable to cope when their loved ones move on or old enemies forget them. At this point, some will become dangerously obsessed with fulfilling those goals, even if the means to fulfill them renders the soul empty: for example, killing anyone who comes near a loved one, in order to keep them from hurting that person. In doing so, the ghost warps into a hollow and, ironically, often makes the focus of their obsession into their first victim, such as Orihime Inoue's brother, Sora, does. This can be prevented if a Shinigami gives the soul a Soul Burial before it descends into a Hollow.

The process by which a soul becomes a Hollow usually takes a year, reflected by the length of the soul's Chain of Fate, which resides in the center of their chest. The Chain of Fate slowly corrodes over time, reflecting the degradation of the soul's ties to the living world. The process can be accelerated under certain conditions, such as being attacked by a Hollow, giving into extreme despair or through the use of certain spiritual powers. When the chain completely decays, they become a Hollow when a hole opens up on their chest, where the chain was attached (signifying that they have lost their heart).

Tendencies
Once a soul becomes a Hollow, it is driven by an eternal and insatiable hunger to devour other souls, living or dead, to fill the void caused by its descent. While their former obsession often becomes their first target, most Hollows will eventually become mindless creatures concerned only with finding the most potent souls to devour. Stronger and more evil Hollows are often able to retain some of their intelligence in Hollow form, making them more dangerous than the average Hollow. While most Hollows are evil, or at the very least mindless, at least a few are not, as is the case with Nel Tu and her Hollow companions, although this could also be because they went through the Shinigamification process. They are also at least partly capable of camaraderie and loyalty, as seen with Grimmjow Jeagerjaques and his companions while they were still Hollows: Grimmjow's subordinates sacrificed parts of their bodies, unasked, for the sake of Grimmjow's growth.

Appearance
Evil souls that vary widely in looks and abilities. Hollows are feared as evil spirits or monsters. Their faces are hidden by strangely shaped masks. These masks are formed to protect the naked instinct that is left in a being after the loss of its heart, and in some ways is the tangible form of the Hollow's madness. The masks of different Hollows can differ greatly in shape and form, but they are always white and skull-like. The masks also obscure the original identity of the Hollows. The mask can be shattered, temporarily allowing the Hollow to regain its original identity, but it will regenerate after a short period once the Hollow takes over again.

Hollows have widely varying physical characteristics and powers, and few of them ever appear the same (the few that do are often clones). In terms of appearance, Hollows can range in appearance from furry (like mammals) through lizard-like to outright demonic. Hollows also vary in size, though it tends to be fairly consistent amongst the various classifications of Menos. In Hueco Mundo, some Hollows are no larger than common pets. Most common Hollows are about twice the size of an adult Human, though it can vary to a degree.

Huge Hollows, aptly named for their size, are as large as two-story buildings. Finally, Gillian-class Menos are even more enormous than the largest of common Hollows, standing as tall as a smaller skyscraper.

Hollows are usually killed by splitting their mask and head in two, but most wounds that would be fatal for a Human will effectively dispatch them. When this is done by a Shinigami's Zanpakutō, the Hollow is not truly killed; its spirit is purified of the sins it committed as a Hollow. This allows the spirit within to enter the Soul Society as a Plus. In cases where a Hollow committed great sins as a mortal, its purification will summon the gates of Hell, through which the soul is dragged to its punishment. When a Hollow is killed by a Quincy, however, it is destroyed, soul and all (presumably, its body becomes spirit particles which disperse afterwards), which has potentially disastrous effects on the spiritual balance of the worlds. The effects of other spiritual powers on Hollows, such as those of Orihime Inoue or Yasutora Sado, are unknown, partly due to the fact that such occurrences are rare.

A Demi-Hollow is an earthbound spirit who is evolving into a Hollow. It usually takes a while for the earthbound spirit to fully transform into a Hollow, but this process can be sped up by forcing the hole in their chest to open. The Demi-Hollows are held down by chains, usually to somewhere they consider to be their territory. They are usually only seen when a Human steps on their territory and cannot be easily sensed by Shinigami.