Those among the Arrancar that excel in killing power are chosen and favored according to their strength are assigned a number from one to ten, collectively referred to as the Espada, holding absolute power and authority over the Números.[1]
At any given time, the Espada has a nominal composition of ten members, all hand-picked and ranked by Aizen himself. Along with Aizen, Gin Ichimaru, and Kaname Tōsen, the Espada as a group possess comparable strength to Soul Society's Gotei 13 Shinigami captains.

At the time of the group's introduction, the Espada 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 (0 being the strongest), originally thought to be ranged from 1 to 10, that indicates their rank and relative power level, which is tattooed somewhere on their body. For instance, Grimmjow Jaegerjaquez's is located on the right side of his lower back and Coyote Starrk's is on the back of his left hand.

The Espada are given free control over the Números and may choose as many of them as they like to become their Fracción. 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. Aizen himself is confident enough of his Espada's abilities to send them out on potentially hazardous missions, such as gathering information in the real world, fighting Shinigami captains inside Las Noches and Karakura Town by themselves, and searching for the Vasto Lorde.

The Espada all together at a meeting called by Aizen.
Added by SantaTeresa
Being Aizen's most direct subordinates (not counting Gin Ichimaru and Kaname Tōsen), the Espada are privy to several of Aizen's plans and most intimate secrets, such as his goals of destroying Karakura Town and creating the King's Key and the location of the Hōgyoku within Las Noches. Aizen's respect for the Espada's abilities is reflected on his manner of addressing them, often referring to them as "my dear Espada" and sitting at the head of the same table with them, while discussing the events around Hueco Mundo. According to Dordoni, Aizen treats them as tools to do his bidding, but nonetheless, he wanted to take back his position as an Espada to once again be close to Aizen. In accordance with this, Aizen is not at all distressed over the deaths of the Espada and calmly congratulates the intruders in Las Noches for their victories against the Espada as he leaves for Karakura Town.
Their immense amount of power and leadership status among the Arrancar also grants each Espada the authority to exert command over Aizen's other forces within Las Noches: the Números, the Exequias, and their own Fracción. There seems to be a sort of hierarchy among the Espada; a higher-ranked member apparently can give orders to lower-ranked members. This is demonstrated when Aizen orders Ulquiorra Cifer to kidnap Orihime Inoue; the Arrancar led by Ulquiorra included Luppi Antenor (the then-sixth Espada), Yammy Llargo (Arrancar 10/ Cero Espada), Grimmjow Jaegerjaquez, and Wonderweiss Margela (unassigned). Various missions usually involve one or more Espada operating together, with the most senior member exercising command over the others; typical, non-combat missions, such as searching for the Vasto Lorde and gathering pre-invasion data in the Human world, include a senior Espada and a lower-ranked one being sent out in pairs. While the lower-ranked members usually have little restraint in taking their own initiative (thus exceeding their orders), higher-ranked members usually stick to Aizen's commands, and are often the first to bail the others out of trouble, because in Nelliel Tu Odelschwanck's own words, they wish to, "prevent the Espada from losing one of their own." However, Aizen's attitude toward the offending Espada members seem to be one of leniency, officially forgiving them (though not without severe sanctions) on the grounds that they are merely advancing his own wishes.

Aside from regularly appearing in Aizen's throne room and the debriefing room, the Espada are known to have their own quarters within Las Noches; those that have been seen appear to have been modified to suit the specific Espada's taste. Szayel Aporro Granz, for instance, possesses a large building that he refers to as his "palace," complete with a hidden, indestructible laboratory and a changing room; Zommari Rureaux has been shown on one occasion to await orders inside a meditation room; Coyote Starrk's room is designed along the lines of a lounging chamber and Aaroniero Arruruerie's tower, accessed via a hanging bridge, effectively keeps out sunlight and contains nooks and crannies for hiding cushions and other implements whose purpose is to make his shape-shifting abilities more convincing.

According to Ulquiorra, any Espada ranked 4th or higher are not allowed to release their Zanpakutō within Aizen's palace, as their Resurrección's released powers are powerful enough to destroy the majority of Las Noches.

History Edit
The creation of the Espada has long been one of Aizen's most important preoccupations. While it is not specified when the Espada were first brought together as a group, the original Espada were already in existence sometime before Aizen's defection from the Soul Society and acquisition of the Hōgyoku. Since then, the composition of the Espada has changed, both in membership and position.
At some point in the past Nnoitra Gilga was the 8th Espada with Nelliel as the 3rd Espada and Szayel Aporro Granz was not a member at all. During this time Aizen frequently sent the Espada out to search for the Vasto Lorde-class Menos.

Before his death, Aaroniero Arruruerie was the sole member of the current Espada who was also part of the first-generation Espada. He was also the only Arrancar from Gillian-class Menos stock to attain a position among the