Hollow

Hollow (虚, ホロウ, horō) 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.

Overview
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.

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,

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 (巨大虚, ヒュージホロウ, hyūji horō), 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.

Destruction
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, which has potentially disastrous effects on the spiritual balance of the worlds.

Demi-Hollow
A Demi-Hollow (半虚, デミ・ホロウ, demi-horō) 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.

Menos Classifications
Though most Hollows are simply transformed Human souls, a sub-group of Hollows known as Menos (メノス, menosu; Spanish for "Minus" or "Less"), exist. Menos are conglomerations of hundreds of normal Hollows, resulting in a single composite entity of far greater strength than any normal Hollow.

Menos are created when the void within an ordinary Hollow's heart becomes so substantial that Human souls are incapable of sustaining it, at which point it begins devouring fellow Hollows. These cannibal Hollows are attracted to one another, and a mass inter-devouring follows, resulting in a Gillian.Following this transformation, the Gillian can evolve twice more, first into an Adjuchas and finally into a Vasto Lorde, provided certain conditions are met. Menos evolution is not only contingent upon the Menos eating other Hollows, but also upon the Menos' ability to avoid being eaten. If another Hollow succeeds in devouring even a part of their body, evolution becomes impossible for the one which was wounded this way. Menos-class Hollows are considered so dangerous that Soul Society sends only highly trained Shinigami teams to handle

them. These incursions seem to be rare, however, as Menos rarely leave Hueco Mundo unless attracted by large sources of spiritual power or encouraged to by other forces.

Gillian
Gillian (最下大虚 (ギリアン), girian; Japanese for "Lowest Great Hollow"), also called Menos Grande (大虚 (メノスグランデ), menosu gurande; Spanish for "Less Big" or "Big Minus", Japanese for "Great Hollow") are the weakest class of Menos and the first stage of Menos evolution. They are giant in comparison to other Hollows, dwarfing even the largest standard Hollows by at least the size of a two-story building, but lack agility and intelligence. They also have long tongues capable of breaking a fellow Hollow's mask. While a Gillian-class Menos is quite powerful, they are still much weaker than a Shinigami lieutenant. They are comparable to "foot-soldiers", reflected in their uniform appearance. In Soul Society, Menos Grande are shown in textbooks discussing Menos.

Though Gillians usually have no personality to speak of, which makes them dangerous since they have no mind of their own, one of the many Hollows that comprise it might gain control over the new entity, reflected in the unique mask it wears in comparison to a normal Gillian. Only these Gillian variants have the potential to evolve into Adjuchas-class Menos. The evolution is facilitated by the continued cannibalization of fellow Hollows, even other Gillians.

Adjuchas
Adjuchas (中級大虚 (アジューカス), ajūkasu; Japanese for "Intermediate Great Hollow") are the middle class of Menos in terms of power, and the second stage of Menos evolution. Adjuchas can vary radically in appearance; anywhere from bulky humanoids to the jaguar-like forms. The are typically much smaller than Gillians, but significantly-larger than a Human. They are also-much smarter than the Gillian-class, and have the power to challenge a captain. It is also likely they could fight evenly with at least a Shinigami lieutenant. They watch over the many Gillian-class Menos and in the anime are also capable of controlling them,

Adjuchas-class Menos are much smaller in population compared to the Gillian class, due to the rarity of any one Hollow being capable of overwhelming the many other Hollows that comprise the body of a Gillian. Furthermore, the Adjuchas must continue to devour Hollows, or its mind will be absorbed by the other Hollows comprising its form and it will revert back into a Gillian. The subsequent lack of an individual will also preclude the reverted Gillian from becoming an Adjuchas again. 

Vasto Lorde
Vasto Lorde (最上大虚 (ヴァストローデ), vasuto rōde; Japanese for "Highest Great Hollow") are the most powerful class of Menos, and the final stage in the Menos evolutionary line. The silhouette of one shown seemed to be very small and humanoid. They are said to surpass an average Captain-Class Shinigami in battle abilities.

Vasto Lorde are the least common form of Menos, but the exact population is unknown.Only a few are known to exist, and are believed to reside within the very deepest parts of Hueco Mundo. Apart from the need to cannibalize even more Hollows, any special requirements for the creation of a Vasto Lorde are unknown.

At least ten of them, under Sōsuke Aizen's control, would be enough to defeat all of Soul Society.

Powers
Like Shinigami, Hollows have a multitude of combat skills and abilities, most of which are usually unique to specific Hollows. The more powerful Hollows, however, have access to a range similar techniques of greater power or use than their own unique skills. Hollows usually acquire energy to power their abilities by devouring souls, but smaller ones can also ingest energy from the atmosphere when in realms with high concentration of spiritual energy, such as Hueco Mundo.
 * Soul-Body Separation: Two middle class Hollow, Shrieker and Acidwire, have been shown using this ability, though presumably all Hollows have the ability. This technique allows a Hollow to forcibly extract a living spirit from his/her body. It is usually not used because Hollows tend to go after Pluses, who have no physical or living body to speak of. The soul is still attached by the Chain of Fate and can be saved, but if it is broken then the soul can prevent the Encroachment so long as he/she is placed inside a soulless body or animal. It seems as though Shinigami have reverse engineered this ability, as shown with the Skull Symbol. It may or may not be possible that this ability could also extract a Shinigami from their Gigai, but this would obviously prove harmful to a Hollow.
 * Acidic Touch: All Hollows possess this power. When a Hollow grabs a Human with their hands, the spot where the person was grabbed is shown to get burned as if acid were poured onto it.. These wounds seem to allow the Hollow's reiatsu to seep into the person, causing great pain to the victim. The wounds can heal if given enough time though, and it is unknown if this ability works on Shinigami or other spiritually powerful beings.
 * Cero (虚閃 (セロ), sero; Spanish for "Zero", Japanese for "Hollow Flash", 'Doom Blast' in the translation) is a highly powerful and relatively swift energy blast that can be fired from various body parts, such as mouth, tongue, hand or fingers. Only Menos, Arrancar, and Vizard have so far been shown to use Cero attacks, and it appears that stronger entities can use it more efficiently and forcefully. Cero also requires some time to be used effectively, so the enemy must be either not moving or busy with something else, or too badly wounded to even simply dodge it, with the exception of a few Cero techniques and users. Additionally, there exists quite a number of variations of Cero, which are usually unique to the Arrancar using it.
 * Garganta (黒腔 (ガルガンタ), garuganta; Spanish for "Throat", Japanese for "Black Cavity") is how Hollows and Arrancar move to and from Hueco Mundo. It literally tears open the dimensional fabric separating the worlds, revealing a tunnel of whirling, torrential energy that must be focused and solidified to create a discernible pathway.
 * Negación (反膜 (ネガシオン), negashion; Spanish for "Negation", Japanese for "Counter-Membrane") fields are used to rescue fellow Hollows. Only the Gillian-class Menos have shown the ability to use it. Espada have a special cube-shaped device called Caja Negación (反膜の匪 (カハ・ネガシオン), kaha negashion; Spanish for "Negation Box", Japanese for "Anti-Membrane of Negation") (Caja de Negación) meant to simulate Negación fields, which are used to trap and punish their subordinates if necessary.
 * High-Speed Regeneration is the ability that allows a Hollow to heal most injuries very quickly, with the exception of the destruction of the head or vital organs. Only Menos-class Hollows have demonstrated this ability. Ulquiorra states that most Arrancar gave up Regeneration for far greater strength.
 * Hollow Shot is a blast that some Hollows have been seen using, it appears to be comprised of spirit particles.

Arrancar
Main article: Arrancar

An Arrancar is a Hollow that has removed its mask, thus gaining Shinigami-like powers. The word Arrancar comes from the Spanish verb meaning "to remove" or "to rip off".

Vizard
Main article: Vizard

The Vizard are a group of Shinigami who have been banished from Soul Society after obtaining Hollow powers.

Trivia

 * Tite Kubo has used a Spanish motif for Hollows and Arrancar throughout the series.
 * A common trait of terms or names used for Hollows is the usage of consecutive letters. Words such Hollow or Arrancar have it, half of the Arrancar and all current and former Espada display it on their names.
 * Demi is the french word for "half", in reference to the name of a "Demi-Hollow" which could mean "Half-Hollow".