The Espada (十刃, (エスパーダ), Esupāda; Spanish for "sword", Japanese for "ten blades") are the top-ranked Arrancar in Ichijin's recently-acquired army of hollow-shinigami hybrids, the most powerful hollows under Ichijin's command, and the main rivals of the Gotei 13.
Outline:
The Espada are regarded as the most powerful arrancar in existence. At any given time, the Espada has a nominal composition of ten members, all hand-picked and branded by Ichijin himself. Along with Ichijin and Various others, 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 1 to 10 (1 presumably being the strongest) that indicates their rank and relative power level, which is tattooed somewhere on their body.
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. Ichijin 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 gathering the Vasto Lorde for his cause.
Their immense amount of power and leadership status among the arrancar also grants each Espada the authority to exert command over Ichijin's other forces within Las Noches: the Números, the Exequias, and their own Fracción. Unlike their Gotei 13 counterparts, there seems to be a sort of hierarchy among the Espada; a higher-ranked member apparently can give orders to lower-ranked members.
History: 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, it is implied that 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. As far as what kind of Arrancar have joined their ranks, only one member of the current Espada was a Gillian-class Menos, the others being at least Adjuchas, so Espada of the past have more prominently ascended from Gillians. Aizen has stated to Gin that his plans for the Espada include filling up its ranks with Vasto Lorde-class Arrancar, a process which he implies in the same statement as ongoing. Before his death, Aaroniero Arruruerie was the sole member of the current Espada who was also part of the first-generation Espada. Nelliel Tu was also the 3rd Espada for a time.
Present Espada
Primera Espada: Ichijin Koumori (-Brandon)
Segunda Espada: Reserved
Tercero Espada: Reserved
Cuatro Espada: Sho Kazamatsuri
Quinta Espada: Reserved
Sexto Espada: Chun Geosung.
Septima Espada:
Octavo Espada: Muerto
Noveno Espada: Reikoku
Decimo Espada:
Aspects of Death
Each of the Espada embody an aspect of death. Those aspects are the ten reasons why humans die. Each Espada governs an aspect of death, which is reflective of that Espada's personality and/or fighting style. It is currently unknown if former and Privaron Espada were once in reflective of any of these aspects, and is also unknown if the aspects are bound by rank (ex: if you're #1, you must govern Solitude).