Hollows 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 which devour the souls of both living and deceased Humans. Hollows settle in Hueco Mundo, but can cross over to the Human World and Soul Society. Hollows are the opposite of Humans.
Hollow are former Pluses (deceased Human souls) who lose their hearts to despair or regret, or they remain in the Human World for too long. Any spirit who is not guided to Soul Society by a Shinigami via Konsō may eventually turn into a Hollow.
The majority of ghosts peacefully move on to the Soul Society after their death. However, some 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 what Orihime Inoue's brother, Sora, does. This can be prevented if a Shinigami gives the soul a Soul Burial before it turns into a Hollow.
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 usually resides in the center of their chest, and connects to their bodies, if present. 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 using 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 they have lost their heart.
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 at least partly capable of camaraderie and loyalty, as seen with Grimmjow Jaegerjaquez and his companions while they were still Hollows; Grimmjow's subordinates voluntarily sacrificed parts of their bodies for the sake of Grimmjow's growth.
All Hollows wear masks, which are formed from the hearts they had lost as Humans. This mask protects the naked instinct which is left in a being after the loss of its heart, and obscures the original identity of the Hollow. The 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, rarely appearing the same. In terms of appearance, Hollows can range in appearance from furry (like mammals) to lizard-like to outright demonic. Hollows vary in size, though it tends to be fairly consistent among 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.
It is revealed by Kūgo Ginjō how Hollows gain their unique appearance and individual powers. When a Hollow's heart is carved out by the hole in their chests, the lost heart becomes its mask and changes its outward appearance to what it looks like as a Hollow. The powers a Hollow wields are manifestations of its heart.
Some Hollows, like White, are artificially created with the same material as Asauchi. These Hollows have the ability to enter a Shinigami's soul and fuse with their Shinigami powers, forming a new Zanpakutō spirit with the abilities of both a Shinigami and a Hollow.
Hollows are usually killed by splitting their mask and head in two, but most wounds which 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, 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 Fullbring users, are unknown, partly because such occurrences are rare.
A Hollow's spiritual power is essentially poison to Quincy, and would destroy their souls completely should it enter their bodies, with no chance of even Hollowfication to occur. It is for this reason that the Quincy sought to destroy Hollows completely, as a philosophy of preserving their own existence.
A Demi-Hollow ("Half-Hollow") is a Jibakurei, 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 a place 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.
Though most Hollows are simply transformed Human souls, a sub-group of Hollows known as Menos (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 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 who was wounded this way. Menos-class Hollows are considered so dangerous 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: Gillian, Adjuchas, and Vasto Lorde.
Gillian ("Lowest Great Hollow"), also called Menos Grande (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" who are generally referred to in textbooks. Gillians are huge, but they are slow and have beast-like intellect. Any captain-level Shinigami can destroy them with ease. Up to the point when Rukia Kuchiki meets Ichigo Kurosaki, she had only seen them in such textbooks, despite having been a Shinigami for many years. 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(Air Gate).
- Appearance: Gillians are huge monster Hollows 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 their 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 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 which 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 member of the Espada that was originally a Gillian-class Menos prior to becoming an Arrancar.
Adjuchas (Japanese for "Intermediate Great Hollow") are the second classification of Menos. They are smaller than Gillians and far fewer in number. They are much smarter and stronger than Gillians, and are 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 which 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 into a Gillian. The subsequent lack of an individual will prevent the reverted Gillian from becoming an Adjuchas again. If any part of an Adjuchas is devoured, they permanently cease evolving and possibly stop regressing into a Gillian, as the Adjuchas in Grimmjow's group asked him to eat them so they would not regress.
Vasto Lorde (Japanese for "Highest Great Hollow") are the third classification of Menos and the highest level of evolution. They are extremely rare in number, and it is said that their entire number within Hueco Mundo can be counted with the fingers on one's hands. The combat capabilities of a Vasto Lorde are above those of the captains of the Gotei 13.
- Appearance: Vasto Lordes are extremely small Hollows, roughly the same size as humans.
- Signature Skills: Cero, Sonído
The process for the creation of a Vasto Lorde is unknown. Grimmjow Jaegerjaquez and his Fraccióneshypothesized that an Adjuchas can become a Vasto Lorde by devouring large amounts of Hollows; however, Shawlong Koufang and his group found this method to be ineffective. After devouring three thousand Hollows, Shawlong speculates that a Vasto Lorde is determined upon an individual's birth as a Hollow, or perhaps before that.
Tōshirō Hitsugaya speculates if Sōsuke Aizen were to gain more than ten Arrancar at Vasto Lorde level, Soul Society would be doomed.
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 ingest energy from the atmosphere when in realms with a high concentration of spiritual energy, such as Hueco Mundo.
- Soul-Body Separation: Two middle class Hollow, Shrieker and Acidwire, have used 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. 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 chased him. The soul is still attached by the Chain of Fate and can be saved, but if it is broken (in Yūichi's case), the soul can prevent the Encroachment so long as he/she is placed inside a soulless body or animal.
- Acidic Touch: Many Hollows, like Shrieker and Acidwire, have displayed this ability, although Rukia implies all Hollows possess this power. When a Hollow grabs a Human with their hands, the spot where the person was grabbed is burned as if acid were poured onto it. This occurred when Orihime was grabbed by the leg, and when Chad's back was wounded. 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, and it is unknown if this ability works on Shinigami or other spiritually powerful beings.
- Cero (Spanish for "Zero", Japanese for "Hollow Flash", 'Doom Blast' in the Viz translation) is a highly powerful and relatively swift energy blast which can be fired from various body parts, such as the mouth, tongue, hand, or fingers. Only Menos, Arrancar, and Visored have used Cero attacks, and it appears stronger entities can use it more efficiently and forcefully. With the exception of a few Cero techniques and users, Cero requires some time to be used effectively, so the enemy must be not moving, busy with something else, or too badly wounded to even simply dodge it. There exist a number of variations of Cero, which are usually unique to the Arrancar using it.
- Garganta (Spanish for "Throat", Japanese for "Black Cavity") is how Hollows and Arrancar move to and from Hueco Mundo. Kaname Tōsen, Kisuke Urahara, and recently Mayuri Kurotsuchi have demonstrated the ability to use the technique. It literally tears open the dimensional fabric separating the worlds, revealing a tunnel of whirling, torrential energy which must be focused and solidified to create a discernible pathway.
- Negación (Spanish for "Negation", Japanese for "Counter-Membrane") fields are used to rescue fellow Hollows. Only the Gillian-class Menos have used it. Espada have a special cube-shaped device called Caja Negación (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 ("Ultra-Fast Regeneration") is an ability which allows a Hollow to heal most injuries very quickly, with the exception of the destruction of the head or vital organs. Ulquiorra states most Arrancar gave up this ability for far greater strength.
Main Article: Arrancar
An Arrancar is a Hollow which has removed its mask, thus gaining Shinigami-like powers
Main Article: Visored
The Visored is a group of Shinigami who have been banished from Soul Society after obtaining Hollow powers.
Main Article: Fullbringer
A Fullbringer is a spiritually aware human who inherited remnants of Hollow powers leftover from an attack on the Fullbringer's parents, prior to birth.
- Tite Kubo has used a Spanish motif for Hollows and Arrancar throughout the series.
- Many Hollow names and related terms contain double letters. The words Hollow and Arrancar, as well as many Arrancar including all Espada and Privaron Espada, have this trait.
- Despite the plural form of the word being spelled as Hollows, the title for Chapter 371 was spelled 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.
- Several Hollows/Arrancar have their Hollow holes in places other than their chests.