Hollow (虚 (ホロウ), horō; literally "Void") are apparitions that are born from the souls of living beings after their death, whom have some strong longing for their world of origin and thus remain bounded to the world of the living, making it impossible to cross over into the Other World. Because they have remained in this unnatural state, their soul begins a descent into corruption as they become strange mask-wearing beasts with a hole through their chests, signifying the loss of their heart and subsequently their lack of life. They all possess supernatural powers that normally incorperate the strengths they had in life in addition to a standard set of abilities Hollows find themselves capable of preforming. These corrupt spirits seek the lives of those around them, acting upon the sole instinct to devour those with a significant level of energy within them, whether they be living or even other apparitions.
Some Hollow attempt to devour one another, and in doing so, undergo radical transformations and evolutions, which are usually unpredictable in nature. The dimension in which they call their home is known as Hueco Mundo. They can be found on any planet in the living physical universe and are considered, in many folk stories, to be the servants of the Makaiōshin themselves.
Hollows are former Pluses (deceased Souls) who lose their hearts and succumb to despair, or are simply left in the World of the Living for too long. Any spirit not guided into Other World by a Shinigami through Konsō will eventually turn into a Hollow.
While the majory of Souls move onto Other World after their deaths, some who stay in the World of the Living for too long, either to fulfill a certain purpose, or because they are simply unguided by Shinigami, will begin to lose their hearts to despair. Due to their limited abilities to interact with the world of matter, this is inevitable, as being a wandering Soul is a poor imitation of life. Those with goals to fulfill 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. When the Soul is warped into a Hollow, ironically the Hollow's first target is always the ones they love. 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 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 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 insatiable hunger to devour all Souls, living or dead. In fact, this is noted to be a sign of impending Hollowfication, if a Soul, which cannot normally be hungry (Souls with high energy however, can feel hunger), are said to be close to Hollowfication if they begin to feel the sensation of hunger. Hollows are usually mindless creatures, however, stronger Hollows are able to retain their individuality (and especially evil Hollows retain their bloodlust beyond the natural capacity for a Hollow), making them more dangerous than the average Hollow. However, several Hollows have some semblance of Humanity, and often stay within Hueco Mundo so they do not need to kill.
The Hollow's most defining trait is it's pure white mask, which shields it's bare instincts from the world. 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. The mask obscures a Hollow's original idenitity, and it is Shinigami protocol to avoid cutting the mask off, so it does not weaken one's resolve to kill a Hollow. 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. Alternatively, removing the mask allows a Hollow to become an Arrancar.
In terms of physical appearance, standard Hollows have great variety, 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.
A Demi-Hollow (半虚 (デミ・ホロウ), demi-horō; lit. "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.
The simplest way to destroy a Hollow is to slice it in half through it's mask, though human wounds can dispatch them as well. When a Shinigami destroys a Hollow through their Zanpakutō, they are not truly killed, but washed of their sins as a Hollow (this is called sublimation) and sent to the Check-In Station to be judged. When a Hollow is killed by a Quincy, it is destroyed, soul and all. The effects of other spiritual powers on Hollows, such as those of Fullbring users, are unknown, partly because such occurrences are rare.
Menos Classifications Edit
While the majority of Hollows are simply transformed souls who have lost themselves and their hearts to despair, there is a sub-group of Hollows known as Menos (メノス, menosu; Spanish for "Minus" or "Less"). 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. To avoid devolving back into a Gillian, higher-level Menos must continue to devour Hollows. However, 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 (and subsequently, de-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.
A common mistake made by those who are uniformed of the process of Menos Evolution is that any Hollow of great power is a Menos, specifically Vasto Lordes. This is false, Menos can only be created by the above described method.
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" 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. 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").
Gillians are huge monster Hollows at least 2-6 stories tall, easily dwarfing even the largest standard Hollow. Their height, however, is the only remarkable thing about their appearance, as all Gillians have the same appearance throughout each one. They all have a white mask with an elongated nose, and only Gillian with the potential to become Adjuachas-class Menos have different masks. Their bodies are covered in what appears to be a billowing black cloak, but is part of the Gillian's body nonetheless, and they possess a ring of white spikes around their neck, in addition to pointed white feet which can be seen under the cloak. 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.
Possessed of great spiritual power, the Gillian-class menos have merely basic Hollow abilities, such as the Cero, and the ability to travel through space using the Kūmon. However, they do have one unique power, the Negación, a special field used to rescue allies.
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
Gillian who become Arrancar have the capacity to feature not just one, but two personalities, though the method this happens is unknown, it can be assumed the Shinigamification used on an Arrancar can reawaken an extra personality, and bring it to life within the newly formed Arrancar.
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. 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.
Unlike Gillians, who bear the same appearance unless they have the potential for further evolution, Adjuchas can vary radically in appearance, anywhere from bulky humanoids to animal-like forms, such as jaguars or centipedes.
Like Gillians, Adjuchas-class Menos are very powerful entities, and can fight on even footing with Captains of the Gotei 13. Like other Hollows, Adjuchas are capable of the Cero, as well as an advanced form of Kūmon known as Garganta. However, the Adjuchas-class Menos have evolved to the point where they begin to develop their own unique abilities that they can use in battle, which makes them a much more formidable threat than the Gillian.
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 prevents 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, permanently locking them in this evolved state.
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 you can count their entire number within Hueco Mundo with only the fingers on your hand. The combat capabilities of a Vasto Lorde are above those of the captains of the Gotei 13
Like the Adjuchas-class Menos, Vasto Lorde vary in appearance, however, they are always fairly humanoid, if retaining a few animlastic traits, and are the small type of large Hollow, around the same size as a human.
Vasto Lorde Hollows possess truly immense spiritual pressure, as it is said the capabilities of a Vasto Lorde is far beyond those of the captains of the Gotei 13, and just ten Arrancar at Vasto Lorde-level would be enough to completely level the Soul Society. Like other the Adjuchas-class Hollows, Vasto Lordes can use the Cero, and Garganta, and also possess unique abilities unto themselves.
Apart from the need to cannibalize even more Hollows, any special requirements for the creation of a Vasto Lorde are unknown.