Arrancar

An arrancar is a hollow that has removed its mask and gained shinigami powers. The name arrancar is a Spanish verb meaning "to remove" or "to rip off," a reference to an arrancar's removal of its hollow mask. The Arrancar become major antagonists during the second half of the series as they start being led by three ex-shinigami captains, Sōsuke Aizen, Gin Ichimaru, and Kaname Tōsen, who serve as Bleach's lead antagonists, especially Aizen.

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 Orb of Distortion, 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 two ways: the location of their holes and the near-total disappearance of their masks. 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 his or her excess power into the form of a zanpakutō.

Unlike with shinigami, this zanpakutō is simply the hollow's true form sealed into a sword. Therefore, an arrancar's release of their zanpakutō is merely a temporary restoration of their former selves. Aaroniero and Nnoitra stated that an Espada's released forms are much more powerful than a Numero's. This act of release is called resurrección (Spanish for "resurrection"). Arrancar do not have a known second release.

Those arrancar under Aizen's command also have a standard theme for their clothing. Their uniforms consist of a white jacket, black sash, white hakama, and black socks with white sandals; in essence, the uniforms are basically inverted shinigami uniforms.

Arrancar ranks
The Espada (Spanish for "sword") are 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.

The Números (数字持ち（ヌメロス）, Spanish for "numbers") are 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).

The Fracción (従属官（フラシオン）, Spanish for "fraction") are 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.

The Privaron Espada (十刃落ち（プリバロン・エスパーダ）,Spanish for "deprived sword") are 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.It was also noted in the manga that Aaroniero Arruruerie was one of the original Espada, but his original number wasn't mentioned since it is possible he was demoted to a lower number in accordance to the arrival of more powerful Espada.

The Exequias (葬討部隊（エクセキアス）,Spanish for "funeral") are arrancar that are part of the execution division, apparently charged with termination of intruders and rebels. They are the equivalent of the Soul Society's Onmitsukidō.

Las Noches
Aizen's arrancar reside in Las Noches (虚夜宮（ラス・ノーチェス）,Spanish for "the nights,"), a massive fortress which can be seen for miles in Hueco Mundo. It consists of a main building with a domed center surrounded by several large towers and smaller buildings. Atop the dome are six smaller towers, which seem to contain areas such as Aizen's throne room and the Espada meeting hall. Like everything else in Hueco Mundo, every building is colored white.

Las Noches is easily the largest building structure in the Bleach universe, appearing as large as (if not larger than) Seireitei in Soul Society. Due to its immense size, Ichigo Kurosaki once commented that it could be an illusion as he, Chad, and Uryū Ishida did not seem to make any progress getting closer to it, despite running until they were exhausted. Also, according to the arrancar Nel Tu, it takes about three days to move from one spot to a gate.

The inside of Las Noches' domes simulate the living world and Soul Society's blue sky, which Aaroniero Arruruerie claims is so the light of the sky will help Aizen observe anywhere he wishes. However, since Aaroniero must stay out of the sunlight to use his abilities, Rukia Kuchiki speculates this is a lie. The position of the halls within the palace can also be adjusted, allowing the one operating them to direct the movements of intruders.

Powers
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 can also ingest energy from the atmosphere when in realms with high concentration of spiritual energy, such as Hueco Mundo.

Bala (虚弾（バラ）,Spanish for "bullet") is an arrancar alternative to the cero. The technique hardens the user's spiritual pressure and fires it like a bullet from their fist. Though weaker than a cero blast, it moves about twenty times faster and can be fired at a much higher rate.

Cero (虚閃（セロ）,Spanish for "zero") are high-powered energy blasts that can be fired from various body parts, such as 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 (王虚の閃光（グラン・レイ・セロ）, guran rei sero?, Japanese for "royal hollow flash," Spanish for "great king zero") and Pesshe Gatiishe and Dondochakka Bilstin's cero sincrético (融合虚閃（セロ・シンクレティコ）, sero shinkuretiko?, Japanese for "united hollow flash," Spanish for "synchronized zero").

Garganta (黒腔（ガルガンタ）,Spanish for "throat") is how arrancar 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.

Hierro (鋼皮（イエロ）,Spanish for "iron") 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.

Negación (反膜（ネガシオン）,Spanish for "negation") 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 Caja Negación (反膜の匪（カハ・ネガシオン）,Spanish for "Negation Box") 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's being used as intended. More powerful arrancar, namely the Espada themselves, can escape given enough time.

Pesquisa (探査回路（ペスキス）,Spanish for "inquiry") is the arrancar equivalent of the shinigami ability to sense spiritual pressure. It functions similar to sonar.

Sonído (響転（ソニード）, Sonīdo?,Spanish for "sound") 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, incontrast to the "swish" sound used for flash steps.