Sōsuke Aizen
Hollows are the class of spirit that most enemies in "Bleach" are a part of. They are spirits that had been human but were not sent to Soul Society in due time after death. This makes them lose their sense of being and gives them a craving for human souls. Their main characteristic is a white mask completely covering their face and a hole near their chest. Each hollow's mask has a design unique to that individual. The Arrancars, Hollows who acquired humanoid form and Soul Reaper powers, become the primary antagonists later in the series under the command of the rogue Soul Reaper Sōsuke Aizen and the ten strongest Arrancars known as the Espadas. The 10 Espadas are numbered 0–9, 0 being the strongest and 9 the weakest. But the rating also works with spirit pressure density numbered 1–10, with Espada's 0's power contained with a 10 tattoo.