The Espada are the top-performing Arracar that were hand-selected by Lord Aizen himself to lead his 'crusade' across Hueco Mundo in Bleach. Thanks to their master's intervention, these Hollows have been embued with Shinigami powers and a Zanpakuto of their own (although their blades don't work quite the same way as the Soul Reapers' do). Sosuke Aizen took it upon himself to assign numbers (ranging from 0-9) to his new subordinates but based his assessments on their reiatsu output (or spirit energy level) alone, without really taking their strength, abilities, or Resurreccion into account.