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 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 Konso may eventually turn into a Hollow.
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 Kugo Ginjo 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.
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 (Japanese for "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 Kumon ("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, Kumon, Negacion.
- 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.
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 Jagerjaques.
- 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.
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.
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