Of all the Arrancar known, the Espada are considered the strongest and those with the greatest destructive capacity, although there are other arrancar whose strength is close to that of some of the Espada. This organization is composed of ten members, all of them selected and elected by Aizen himself, and together with him, Gin Ichimaru and Kaname Tōsen are the most responsible for the enemy army to the Shinigami of the Soul Society. In addition, their level of power and capabilities are comparable to those of the Captains of Gotei 13, which makes them fearsome enemies, surely the most dangerous of all those that can be found in the Hueco Mundo.

At the time of its introduction the group of the Espada is made up of nine men and one woman, all of whom are assigned a number from 1 to 10 based on their strength, indicating their rank and relative power level. This number, written in Arabic characters, is also tattooed on any part of the body of the selected one. The Espada have total control over the rest of the Arrancar, the Numbers, and they have the prerogative of choosing as many as they want to become part of their exclusive service, becoming their Fraction. However, in the same way that there is a great difference in level between Shinigami Captains and Sub-Captains, the same can be said of Espada and any member of their Fraction or Numbers.