Arrancar

An is a Hollow that has removed its mask and has gained Shinigami-like powers. The name "Arrancar" is a Spanish verb meaning "to remove" or "to rip off" (the kanji used means "ripped mask"), a reference to an Arrancar's removal of its Hollow mask. Following the retrieval of the Hōgyoku and Aizen's emergence into Hueco Mundo, they have become the main antagonists of the series, in particular the Espada, Aizen's top ten most powerful Arrancar.

Overview
Naturally occurring Arrancar do exist within Hueco Mundo. 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 Shinigamification.

Arrancar and ordinary Hollows differ in several ways:


 * Humanoid Form: Arrancar assume a humanoid form, as opposed to the animal-like or monstrous forms seen with Hollows. Upon becoming an Arrancar, the chances of a Hollow gaining a humanoid form increase with their intelligence; only Vasto Lorde-class Menos will definitely assume a humanoid form. This new form will retain remnants of their past selves, like fragments of their removed masks and a Hollow hole. While some Arrancar (e.g. Nnoitra Jiruga and Tia Harribel) prefer to conceal these remnants, others choose to hide their faces completely to escape unwanted attention. Most also have a colored marking of some sort on their face, though some only gain these markings upon the release of their Zanpakutō (Ex: Nnoitra, Szayel Apporo, and Harribel). A few cases that include marks visible without the release of a Zanpakutō include Ulquiorra's tear marks, Sun-Sun's pink dots, the dashes under Grimmjow's eyes, and the marking which runs across Nelliel's cheekbones and nose.


 * Zanpakutō/Soul Separation: Arrancar can gain a Zanpakutō although some Arrancar's Zanpakutō don't take the shape of a sword. Unlike with Shinigami, this Zanpakutō is not sentient, but is simply their "true" Hollow form sealed into a sword. Therefore, an Arrancar's release of their Zanpakutō is merely a temporary restoration to their former selves, granting them any powers their Hollow form possesses. The act of release is called Resurrección (Spanish for "Resurrection"), which is activated with a command phrase followed by the name of the sword. The Resurrección of an Espada-level Arrancar releases enough power to be comparable to a Shinigami's Bankai. Few Arrancar do not have a Zanpakutō, being too weak to seal their excess power into a blade, while others prefer to have it constantly released. In the case of Starrk and Lilynette, their Shinigamification separated them into two different souls, rather than an Arrancar and a Zanpakutō, so Starrk releases by touching Lilynette and speaking his release. Although, recently it is suggested through flashback that Starrk and Lilynette separated prior to their Shinigamification, making the means by which their powers were sealed more ambiguous.

The Arrancar under Aizen's command have a standard theme for their clothing. Their uniforms consist of a white jacket, black sash, white hakama, and black and white boots that bear an opposing similarity to a Shinigami's sandals; in essence, the uniforms are basically inverted Shinigami uniforms.

While most of the Arrancar either develop or retain their own personality traits, the majority (not all) of Arrancar shown in the series appear to be similarly cold, cruel, ambitious, impatient, selfish, overconfident, ruthless, arrogant, impulsive, brutal, relentless, battle-hungry, overbearing, short-tempered, indifferent, and/or egotistical.

Though not elaborated upon as of late, not all Arrancar residing in Hueco Mundo are loyal to Aizen. They prefer not to interfere with his plans, presumably because of Aizen's immense amount of power and army of servants. It is for that reason many naturally-evolved Arrancar, namely those that relish battle (like Nnoitra), felt it necessary to align themselves with him to obtain more power. While some Arrancar choose to follow Aizen in the hopes he will deliver them salvation, others ally with him purely out of respect. As Iceringer put it, Aizen is someone who is fearless, which creatures born out of fear and pain, such as Hollows, find admirable.

Although Arrancar are Hollows who have gained Shinigami powers, most still refer to themselves as Hollows (such as Ulquiorra, Zommari, Grimmjow, etc.)  This is similar to how Ichigo Kurosaki still refers to himself as a Shinigami despite his steady evolution into a Vizard.

Espada
The Espada are the top-ranked Arrancar in Aizen's army, and the primary antagonists in the later half of the series. 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 0 to 9 (with 0 being the strongest, while 9 being the weakest) that indicates their rank and relative power level, which is tattooed somewhere on their body (Grimmjow's is on his lower back, for instance). Each Espada embodies an aspect of death, which ties into their personalities, beliefs and powers. 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, speed, intelligence, and power 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. Nelliel Tu Oderschvank was formerly the 3rd Espada before she was tricked and exiled by Nnoitra.

Privaron Espada
The Privaron Espada are former Espada who have officially lost their rank and are assigned a three-digit number. Being former Espada, they are far stronger than normal Números. They reside in the part of Las Noches called Tres Cifras As mentioned by Dordonii Alessandro Del Socacchio, these former "naturally-created" Espada were rendered obsolete after Aizen began artificially creating Arrancar to meet his own standards. It is also noted that Aaroniero Arruruerie was one of the original Espada, but his original number was not mentioned since it is possible he was demoted to a lower number in accordance to the arrival of more powerful Espada.

Números
These Arrancar are the combat specialists. They are assigned a two digit number at birth from 11 to 99 to indicate their age (Número 11 being the oldest, while 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
The Fracción are a subsection of the Números that directly serve a specific Espada. The number of Fracción for any given Espada varies: Starrk and Nnoitra Jiruga seem to have just one, while Szayel Aporro Granz has at least a dozen. There are a few Espada that apparently prefer to fight alone, such as Yammy Riyalgo, Zommari Leroux, Ulquiorra Cifer and Aaroniero Arruruerie, who seem to have chose not to have any Fracción under their command.

Exequias
The Exequias are Arrancar that are part of the execution division, apparently charged with termination of intruders and rebels. They are the equivalent of Soul Society's Onmitsukidō. Their captain is the bull-headed Rudobōn, who can spawn a seemingly infinite amount of the soldiers that form this group using his Zanpakutō.

Las Noches
Aizen's Arrancar reside in Las Noches, a massive, white-colored fortress that resides in the center of the Hueco Mundo desert. When Barragan was King of Hueco Mundo, it was nothing more than a large throne and a few pillars. It wasn't until Aizen took over that the fortress became more elaborate and complete. It is shaped like a large dome with various towers and stands surrounding it, many leading to underground passageways. The inside is filled with huge cylinder-shaped buildings, many of which are destroyed during battles between Ichigo and the Arrancar. The inside of the dome has a day sky painted on its surface, making it seem like a blazing hot desert (even though it's always night in Hueco Mundo). In the center of the fortress is the building where Aizen and his Espada spend most of their time, often having meetings across a long table.

Powers
Like Shinigami, Arrancar have a multitude of combat skills and abilities, some of which are similar to those of the former, and some of which are variations on ordinary Hollow powers.


 * Bala is an Arrancar alternative to Cero. The technique hardens the user's Spiritual Pressure and fires it like a bullet. Although it is weaker than a normal Cero, it moves about twenty times faster and can be fired at a much higher rate, but the damage is much the same. Stronger Arrancar can also fire a more powerful Bala, while retaining its twenty-fold speed.


 * are high-powered energy blasts that can be fired from various body parts, such as mouth, tongue, hand or fingers. Only Gillians, Adjuchas, Arrancar, and the Vizards 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, the Cero Oscuras that Ulquiorra Cifer can use only in his released form, as well as the Cero Sincrético performed by Pesche Guatische and Dondochakka Bilstin.


 * is how Arrancar and Menos-class Hollows move to and from Hueco Mundo. Kaname Tōsen and Kisuke Urahara have also demonstrated the ability to use the technique. 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.


 * 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 generally have proportionally stronger skin than usual.


 * are energy fields are used to rescue fellow Hollows. Only the Gillian-class Menos have shown the ability to use it. Negación fields surround a target in a square beam of light that isolates the target from the dimension they are currently in, making it impossible to harm them. The field then pulls the target towards the Menos that created the field. Espada have a special cube-shaped device called meant to simulate Negación fields, which are used to punish their subordinates if necessary. The cube, when used on a subject, traps them in an alternate dimension from which there is no escape; however, this assumes it is being used as intended. More powerful Arrancar, namely the Espada themselves, can escape given enough time.


 * is the Arrancar equivalent of the Shinigami ability to sense Spiritual Pressure. It functions very similarly to sonar.


 * Most Arrancar have the ability to seal their power within a Zanpakutō. Upon releasing their Zanpakutō, they gain all of their former unique Hollow abilities back, essentially 'resurrecting' their old selves. When using this ability, they essentially combine their human form and their Hollow form, incidentally increasing their power considerably. As another note, when an Arrancar uses his or her Resurrección, it heals all minor wounds.


 * is the Arrancar equivalent of the Shinigami Flash Steps and the Quincy Hirenkyaku; it allows the user to travel at incredibly high speeds for short distances. Use of Sonído is punctuated by a brief static sound, in contrast to the "swish" sound caused by Flash Steps.

Trivia

 * In the English version, they refer to more than one Arrancar as "Arrancars," but in the original Japanese version, "Arrancar" is already plural, because in japanese some words have the same form in both plural and singular.