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.

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. 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. They are also at least partly capable of camaraderie and loyalty, as seen with Grimmjow Jaegerjaquez 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.

, 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. 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.

Demi-Hollow
A is a Jibakurai, 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, 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. There are three categories of Menos, the first is Gillian, the second Adjuchas, and Vasto Lorde.

Gillian
, also called are the first and weakest of all Menos. In human terms they would be foot soldiers. There are many of them and they all look alike. In Soul Society, they are the "Menos" that are generally referred to in textbooks. Gillians are huge, but they are slow and have beast-like intellect. Any Captain would be able to destroy them with ease. Up to the point when Rukia Kuchiki meets Ichigo Kurosaki, she has only seen them in such textbooks, despite having been a Shinigami for many years. Gillains commonly attack in a group and one is powerful enough to be a challenge for a Lieutenant of the Gotei 13.


 * Appearance: Gillians are huge monster Hollows towering at least 2-6 stories tall, easily dwarfing even the largest standard Hollow. They all have a white mask with an elongated nose. Their bodies are covered from theirs head to their feet with a black covering reminiscent of a cloak. Around their neck is a row of thin white spikes. They have large, pointed feet wrapped in white bandages. They have large white hands with long, sharp, white nails, but they are commonly hidden within their cloak, rarely revealed even in combat. They also have long tongues capable of breaking a fellow Hollow's mask.


 * Signature Skills: Cero, Garganta, Negación


 * Evolution: 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 newly formed 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.

Aaroniero Arruruerie is the only originally Gillian-class Menos to have achieved the rank of Espada after his Arrancarification.

Adjuchas
are the second classification of Menos. They are smaller than Gillians and far fewer in number. However, they're much smarter and stronger than Gillians. They're often the ones who lead Gillians into battle. Adjuchas are commonly solitary creatures rarely joining into groups. They are powerful enough to challenge a captain of the Gotei 13.


 * Appearance: Adjuchas can vary radically in appearance; anywhere from bulky humanoids to the jaguar-like form of pre-Arrancar Grimmjow Jaegerjaquez.


 * Signature Skills: Cero, Garganta, Unique Individual Powers & Abilities


 * Evolution: 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
are the third classification of Menos and the highest level of evolution. They are extremely small Hollows, roughly the same size as humans and are extremely rare in number. It is said that you could count their entire number within Hueco Mundo with only the fingers of your hand. The Vasto Lordes' combat capabilities are above that of the captains of the Gotei 13.


 * Appearance: Unknown.


 * Signature Skills: Unknown.

Apart from the need to cannibalize even more Hollows, any special requirements for the creation of a Vasto Lorde are unknown.

Tōshirō Hitsugaya states that if Sōsuke Aizen has more than ten Arrancar at Vasto Lorde level, Soul Society would be doomed. Of the Espada, Harribel has been confirmed to being a Vasto Lorde at the time of her recruitment.

In ichigo vs ulquiorra, ichigo is killed by ulquiorra's cero but revived by hollow ichigo thus turning ichigo into a vasto lorde and fighting ulquiorra and cutting his arm off but regenerated after words after showing off his power and defeated ulquiorra, Vasto lorde ichigo reverted back to normal with half of his mask smash after his cero blowed up by ulquiorra cutting off ichigo's horn

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. Acidwire used this on his younger sister, Orihime, so she could see and converse with him. Shrieker used it for more sinister means, causing the young boy Yūichi Shibata to be trapped within the body of a parakeet for three months while the Hollow chases him. The soul is still attached by the Chain of Fate and can be saved, but if it is broken (in Yūichi's case) 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: Many Hollows, like Shrieker and Acidwire, have displayed this ability, although Rukia implies that 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. This was shown when Orihime was grabbed in the leg, and on the wound on Chad's back. 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.


 * 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.


 * is how Hollows and Arrancar move to and from Hueco Mundo. Kaname Tōsen, Kisuke Urahara and recently Mayuri Kurotsuchi have also demonstrated the ability to use the technique. 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.


 * 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 de Negación in the English dub) 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.


 * Unnamed Hollow Blast is a blast that has been seen used by a Hollow. It is thinner than a cero, but quicker.

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
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 as Hollow or Arrancar have it, and half of the Arrancar and all current and former Espada display it on their names.
 * Despite the plural form of the word being spelled as Hollows, the title for chapter 371 is worded as Kingdom of Hollowes when the chapter first appeared. However, the title of chapter 371 was subsequently changed to Kingdom of Hollows when it appeared in Volume 43 - which was also called Kingdom of Hollows.