Every shonen anime needs villains for the heroes to fight. In the world of Bleach, those villains are the Arrancar, or unmasked hollows. But there is more to them than meets the eye.
Arrancar are a sort of Soul Reaper/hollow hybrid, and until Sosuke Aizen came along, very few of them existed, and even fewer were a serious threat for any Soul Reaper that encountered them. Aizen is a resourceful villain, however, and he knew that the Soul Society alone couldn't provide everything he needed to realize his master plan. So, he turned to the vast potential of hollows, and unlocked power that has long since been impossible to seize. Thus, the Arrancar became the primary antagonists in the later arcs, commanded by Sosuke Aizen himself.
Arrancar: Powers and Abilities
Arrancar don't have access to kido spells, shikai or bankai like Soul Reapers do but they do have a number of abilities unique to their race. Some are inherent to hollows, and others appear unique to Arrancar in particular. Many of them are rough analogs for what Soul Reapers are capable of.
An Arrancar can use pesquisa (Spanish for "research"), a sensory ability that allows an Arrancar to detect and measure the strength, nature and location of another being's spiritual pressure. This even works from a distance; Grimmjow used it to sense a handful of beings all over Karakura Town one night, and with that information, he sent his six fracciones (minions) after Ichigo and his friends. Ulquiorra has a sharply refined pesquisa but the hulking Yammy struggles to use it at all, and can only guess how powerful his opponents are.
Arrancar are quick but they can't perform flash step. Instead, they have a similar ability known as sonido (Spanish for "sound"), which even low-ranking Arrancar can use, such as Shawlong Koufang. c, the seventh Espada, can use such fast sonido: he creates afterimages that serve as decoys in combat. He calls it "gemelos sonido" or "sound twins."
On the defense, Arrancar have a very hard skin known as hierro (Spanish for "iron"), which is not easy to cut or break. With this hard skin, an Arrancar can shrug off sword blows from low-ranking Soul Reapers and even touch sharp swords bare-handed. Both Grimmjow and Nnoitora were seen seizing Zangetsu with their bare hands, and Nnoitora has the hardest hierro of all. And in Ichigo's final duel with Ulquiorra, a mighty sword stroke only put a short, shallow cut on Ulquiorra's skin.
Arrancar have a number of offensive abilities, the main one being cero ("zero") blasts. Like regular Menos, an Arrancar can charge up its spirit energy at one spot (such as a hand or the mouth), and discharge it in a wide cone. This technique is slow so some Arrancar like to use bala blasts ("bullet"). These don't have the same destructive energy, but they are 20 times faster and are useful against quick targets.
There is more than one way to fire a cero. Espada can charge up a stronger version known as a Gran Rey Cero, as Grimmjow once demonstrated. It requires a small blood sacrifice and once ready, it can fire a devastating blast of energy. Meanwhile, some Espada (mainly Ulquiorra) can fire the Cero Oscuras ("black zero"), which has the greatest destructive potential of any cero blast.
Arrancar are hybrids because they have humanoid bodies and intelligence -- not to mention a zanpakuto. But these swords are not the living entities that Soul Reapers use. Instead, an Arrancar's zanpakuto is a sword-shaped container for the user's original hollow body, complete with a name and a release command (typically, a Spanish name).
These mutant zanpakuto can be used as conventional swords, but to access their full power, an Arrancar will state their release's name, and their zanpakuto disappears. In its place, that Arrancar has their original hollow body largely restored, including its powers. Most Arrancar still have their humanoid shape, while their original hollow body acts like armor (such as Grimmjow's panther suit or Ulquiorra's bat wings). In addition, the Arrancar's mask remains will change and expand, more closely resembling that of their original hollow body.
This is an Arrancar's resurrection and they can undo it at will to restore their zanpakuto. Other times, as seen with Grimmjow, extreme injury and fatigue will cause the resurrection to undo itself since there's not enough energy to sustain it any longer. And if an Arrancar removes a part of their body while in release state, that body part is permanently lost, as Cirucci Sanderwicci once demonstrated by removing her moth wing feathers. Otherwise, release state abilities vary widely, and most are unique. Grimmjow and Nnoitora receive melee combat bonuses, while Zommari Rureaux can control another person's body parts by observing them with his enchanted eyes. Some Arrancar can also fly when they perform a release, such as Cirucci and Abirama Redder.
The Arrancar And Aizen
Bleach's traitor Captain Sosuke Aizen did not invent the concept of Arrancar but he did realize their potential. He's not doing this to "save" hollows or give them a future, however. They are tools to him -- or even research subjects -- but nothing more than that. Many Arrancar worship Aizen as a savior or king but he is nothing of the sort.
As stated earlier, Arrancars have always existed, albeit in limited numbers, and their spiritual pressure tended to be weak and unstable. So, what changed? In his quest for supreme power and to rule all of creation from the heavens, Aizen needed a lead that the Soul Society couldn't provide. As he once told Ichigo, a given skill or power can only be enhanced to a certain degree before being mastered completely, and Aizen despaired at these upper limits. He found a way to shatter those barriers: by combining Soul Reapers and hollows and breaking down the barrier between them.
Both Aizen and Kisuke Urahara developed small spheres called Hogyoku, with Kisuke's being more effective. Aizen later stole Kisuke's, used it on the many powerful Menos he found in Hueco Mundo (the hollow world) and then used that sphere's incredible power to make strong Arrancar. One by one, he exposed hollows to that sphere and shattered the masks of powerful Gillian, Adjuchas and even Vasto Lorde class hollows. He assigned them ranks and uniforms and allowed the ten Espada to choose fracciones to serve under them.
But even all this can't save the hollow race. As Nnoitora once said in a flashback, "There's no salvation for our kind," and he was right. Becoming an Arrancar gives a hollow its human reason and form back, as well as boosted power, but this will never give hollows a comfortable place in the world. They are doomed to be the enemies of the Soul Reapers and Quincy alike, and becoming better-armed won't change that.
In this way, they are a mirror of Aizen's mad plan: an aggressive but fatally flawed quest for supreme power, fueled by denial and despair. Aizen despaired at the lack of an all-powerful figure in the heavens, and hollows despair at their bloody lot in life. No matter how much power Aizen or his Arrancar minions gain, what they yearn for is impossible.