Info on Hollows

Hollows are former pluses (deceased souls) that lose their hearts to despair or remain in the real world for too long. Any spirit that is not guided to Soul Society by a Soul Reaper may eventually descend into a hollow. The process by which a soul becomes a hollow usually takes months (if not longer), reflected by the length of the soul's Chain of Destiny, which resides in the center of their chest. The Chain of Destiny 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 another hollow or through the use of certain spiritual powers. When the chain completely decays, the soul is warped into a unique and monstrous form with a vaguely skeletal mask and a large hole where their chain was once connected.

Hollows have widely varying physical characteristics and powers, and few hollows ever appear the same (the few that do are often clones). In terms of appearance, hollows can range from furry (like mammals) or lizard-like in appearance to outright demonic. A hollow's abilities are often determined by their power. Weaker (and by definition less intelligent) hollows usually only attack in a melee style, whereas stronger hollows have a wide array of abilities with which to kill their quarry. Hollows also vary in size, though it tends to be fairly consistent amongst the various classifications of hollows. 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. Huge hollows, aptly named for their size, are as large as two-story buildings. Finally, gillian-class menos are even larger than the huge hollows, standing as tall as a common skyscraper.

Menos
Though most hollows are simply transformed human souls, a sub-group of hollows known as menos, exist. Menos are conglomerations of many normal hollows (from hundreds to thousands, depending on the type), resulting in a single composite entity of far greater strength than the hollows that spawned it.

Gillian
the weakest class of menos and the first stage of menos evolution. They are giant in comparison to other hollows, dwarfing even the largest standard hollows by at least the size of a two-story building, but lack agility and intelligence. While a gillian-class menos is quite powerful, they are still much weaker than a Soul Reaper lieutenant. According to Tōshirō Hitsugaya, they are comparable to "foot-soldiers", reflected in their uniform appearance. In Soul Society, gillians are known as Menos Grande and are shown in textbooks discussing menos.

hough gillians usually have no personality to speak of, one of the many hollows that comprise it may retain control over the new 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, resulting in a unique but still massive form before they completely become adjuchas.

Adjuchas
the medium class of menos in terms of power, and the second stage in the menos evolution. Adjuchas can vary radically in appearance, from bulky humanoids to the panther-like form of Grimmjow Jeagerjaques. Unlike the gillians, they are roughly human-sized. They are also much smarter than the gillians and have the power to match. It is likely they could fight evenly with at least a Soul Reaper lieutenant. They watch over the many gillian-class menos.

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 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
the most powerful class of menos, and the final stage in the menos evolutionary line. They appear almost completely humanoid, and each one is stronger than an average Soul Reaper captain.

Vasto lorde are the least common form of menos, but the exact population is unknown. They are believed to reside within the very depths of Hueco Mundo. Apart from the need to cannibalize even more hollows, any special requirements for the creation of a vasto lorde are unknown.

Arrancar
An arrancar is a hollow that has removed its mask and gained Soul Reaper powers.Few hollows have the potential to remove their masks normally, and even if they manage it the change is not significant. Sōsuke Aizen is able to artificially create arrancar with the Hōgyoku, which also unlocks much more of their potential than a natural transformation would. The process of turning a hollow into an arrancar is called soulreaperification. Arrancar differ from ordinary hollows in two ways: their hollow holes are often moved, and only a small fragment of their masks remain, along with a mask pattern (Such as Ulquiorra's tear marks or Yammy's cheek marks). When hollows become arrancar, the chances of them retaining a humanoid form increase with their intelligence; only vasto lorde-class arrancar will definitely assume a humanoid form.

If powerful enough, an arrancar can seal its excess power into the form of a zanpakutō. This weapon is their original hollow powers, which can be called upon when necessary, as opposed to the living spirit of a Soul Reaper's zanpakutō. Their zanpakutō also differ in that they need not take the form of a sword: for example, Cirucci Thunderwitch's weapon takes the form of what is essentially a giant yo-yo. When an arrancar releases their zanpakutō, a process called resurrección, part of their original hollow form is restored, giving them access to their original abilities. While arrancar do not appear to have a second release, the act of releasing their zanpakutō exponentially increases their power, similar to the power boost afforded by a Soul Reaper's bankai.

Arrancar ranks

Espada
the top-ranked arrancar in Aizen's army. The Espada at the time of the group's introduction consists of nine males and one female.Each Espada member is chosen for their superior combat ability and then assigned a number from 1 to 10 that indicates their rank and relative power level, which is tattooed somewhere on their body (Grimmjow's is on his lower back, for instance). The Espada are given control over the Números and are regarded as the strongest arrancar in existence. Similar to the power gap between captains and lieutenants in the Gotei 13, the strength of the Espada far surpasses that of the average Número. As Aaroniero Arruruerie notes, all of the Espada except him were at least adjuchas-class menos before becoming arrancar.

Números
the combat specialists. They are assigned a two digit number at birth from 11 to 99 to indicate their age (Número 11 is the oldest, Número 99 is the youngest). The basic Número uniform consists of a white-collared jacket and robes held with a black sash. However, the uniform is customizable, giving each Número a sense of individuality. The fighting ability of Números is thought to be superior to all other arrancar, except for the Espada (current and former).

Fracción
a subsection of the Números that directly serve a specific Espada. The number of Fracción for any given Espada varies: Stark and Nnoitra Jiruga would seem to have just one, while Szayel Aporro Granz has at least a dozen.

Privaron Espada
former Espada who have officially lost their rank and are assigned a three-digit number. Being former Espada, they are far stronger than the normal Números. They reside in Tres Cifras, which translates literally from Spanish as "three digits". As mentioned by Dordonii, these former "naturally created" Espada were rendered obsolete after Aizen began artificially creating arrancar to meet his own standards.

Exequias
arrancar that are part of the execution division, apparently charged with termination of intruders and rebels.

Combat

Bala is an arrancar alternative to the cero. The technique hardens the user's spiritual pressure and fires like a bullet. Though weaker than a cero blast, it moves twenty times faster and can be fired at a much higher rate.

Cero are high-powered energy blasts that can be fired from various body parts, such as the 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. Various arrancar and hollow characters display variants of the technique, such as the Espada-exclusive gran rey cero.

Hierro refers to the hardened skin of the arrancar, which is a result of their compressed spiritual power. While their skin is strong enough to block even released zanpakutō bare-handed, it is by no means impenetrable. Stronger arrancar have proportionally stronger skin.

Sonido is the arrancar equivalent of the Soul Reapers' flash steps and the Quincy hirenkyaku; it allows the user to travel at incredibly high speeds for short distances.