An Arrancar (arankaru) 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.

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 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.