In the Bleach universe, Hollows are nearly always villains. They are the tormented souls of the dead, bound to the earth by unfulfilled desires and starving for other souls. The holes in their bodies represent that emptiness, and they sometimes retreat to Hueco Mundo to devour each other and avoid the Soul Reapers. But some Hollows have become much more.
Gillian-class Hollows are amalgamations of many other Hollows, and they can evolve into Adjuchas or Vasto Lorde-class Menos Grandes. If their masks are removed, they may gain enough power to become an Espada, the 10 strongest Arrancars of them all.
The Hollow Captains
Many Arrancars were born when Sosuke Aizen exposed many Hollows to the Hogyoku as a part of his overall scheme. Most Arrancars fall short of what a Soul Reaper Captain is capable of, but several Adjuchas and Vasto Lorde-type Hollows rose to the top, and Aizen made sure to give them special ranks and rights among the Arrancar. Most Arrancars have an official number to mark the order in which they were born, but the Espadas are ranked according to strength instead, from one to 10. These numbers are tattooed in temporary ink to mark their position, and these numbers are liable to change. If a new Arrancar is born and is Espada-worthy, then the weakest Espada is ejected from the group to make room for the newcomer. When that happens, some of the Espada will have their ranks changed to fit the new member. For example, Nnoitora Gilga was once Espada #8, but during the main events of Bleach, he was #5.
These Arrancars are meant to be Sosuke Aizen's equal to the 10 remaining Soul Reaper Captains. Some of the Espadas actually outclass the Captains in terms of power, mainly the Vasto Lorde ones such as Barragan Luisenbarn (#2) and Tier Halibel (#3). The Espadas ranked #4 and up are not supposed to release their zanpakuto inside Las Noches (Aizen's fortress), since the overwhelming power can actually damage the fortress a great deal. Ulquiorra Schiffer, Espada #4, took, the time to reach Las Noches' outside roof before releasing his zanpakuto, Murcielego.
The 10 Espadas all answer to their general, former Captain Kaname Tosen, and they to Sosuke Aizen, their supposed savior (though there is no true salvation for Hollows, as Nnoitora realized). These Espadas are allowed to have their own minions, the fracciones. These lesser Arrancars may be hand-picked by the Espadas to serve them, and Grimmjow, as an example, had five fracciones. They had been his companions during their Hollow days, and they were all fiercely loyal to him. Barragan had even more fracciones, a full squad of six. And that's not the only reward the Espadas get for their loyal to Aizen; each one also has a miniature palace to themselves inside Las Noches, and these palaces may be unique to suit each Espada's personal tastes or needs. Szayelaporro Grantz, Espada #8, used his palace as a laboratory.
All of the Espadas are powerful and demand the full attention of the Soul Reapers, but even then, a few of them stand out. For example, Espada #9, Aaroniero Arurruerie, is actually a Gillian-class Menos, a being that ordinarily lacks the strength to have that rank. But he is the embodiment of gluttony, being able to eat other Hollows and obtain the full measure of their strength and abilities. He has eaten 33,650 Hollows, and their combined power was Espada-worthy. Still, he seemed to command little respect among the others, since they all sneered at him when he was killed at the hands of Rukia Kuchiki (he had been imitating Kaien Shiba, Rukia's old friend).
Nelliel Tu Oderschwank was once Espada #3 before the main events of Bleach, but she was betrayed. Nnoitora, who had a bitter grudge with her, conspired with Szayelaporro to ambush her and injure her head. As a result, her spiritual energy leaked out, and she shrank into a small child. Nelliel was abandoned in the Hueco Mundo desert, and the Espadas were shuffled around. Later, Nelliel returned as Ichigo's friend, and she temporarily regained her powers and memories. Now she is hostile to her former comrades, primarily Nnoitora.
Ulquiorra Schiffer is the poster boy of the Espadas, being powerful, disciplined and mission-oriented. He is Espada #4, and Aizen typically trusts him with the most important missions, such as kidnapping Orihime and scouting out Ichigo's current power level in Karakura Town. For that reason, Ichigo mistook him for the leader of the Espadas. Ulquiorra is unique for another reason: he has two releases. His main release is Murcielego, which grants him two large horns and bat wings, but he has a hidden second stage, known as Segunda Etapa. It turns Ulquiorra into a slender creature with a black body and a long, thin tail, and his power levels increase even more. It is possible that if Aizen knew about that form, Ulquiorra would have been ranked higher.
Yammy Llargo is ordinarily ranked #10 among the Espadas, but when he releases his zanpakuto, Ira, his power can grow indefinitely according to his anger (and he grows larger, too). His rank converts to zero in this form, making him the strongest Espada of all if his anger reaches sufficient levels. Still, he lost soundly to the Captain Byakuya Kuchiki/Captain Kenpachi Zaraki tag team.