Hollow

Spoiler:

Hollow

Hollows are former Pluses (deceased Human souls) that lose their hearts to despair or remain in the real world for too long. Any spirit that is not guided to the Soul Society by a Shinigami via Konsō 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 and 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.

The process by which a soul becomes a Hollow usually takes months to years to complete, 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, coinciding with 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

Fishbone D, the first Hollow seen in the series.
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
All Hollows wear masks, which are formed from the hearts that they had lost as Humans. This mask protects the naked instinct that is left in a being after the loss of its heart, and to obscure the original identity of the Hollow. he masks of different Hollows can differ greatly in shape and form, but are always white and skull-like. 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 few of them ever appear the same. 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.

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.

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.

hese 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 lastly Vasto Lorde.

Gillian

Gillian-class Menos.
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 first and weakest of all Menos. In Human terms they would be considered 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. Gillians commonly attack in a group and one is powerful enough to be a challenge for a Lieutenant of the Gotei 13. Gillians typically arrive in the Human World through the use of spatial distortions called Kūmon (空門, lit. "Air Gate").

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, Kūmon, 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.

Adjuchas

Adjuchas-class Menos
Adjuchas (中級大虚 (アジューカス), ajūkasu; Japanese for "Intermediate Great Hollow") 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.

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

Tier Harribel as a Vasto Lorde.
Vasto Lorde (最上大虚 (ヴァストローデ), vasuto rōde; Japanese for "Highest Great Hollow") 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.

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. 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.

Acidic Touch: Many Hollows, like Shrieker and Acidwire, have displayed this ability, although Squad 12 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.

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 Viz 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 Visored 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 in the English dub) meant to simulate Negación fields, which are used to trap and punish their subordinates if necessary.

High-Speed Regeneration (超速再生, Chōsoku Saisei; lit. "Ultra-Fast 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. Ulquiorra states that most Arrancar gave up Regeneration for far greater strength.