Shinigami are souls that have supernatural powers. As such, their bodies are composed of Spiritrons (spirit particles) instead of atoms. This means that just like most other types of spirits, they can only be seen by other spiritually aware entities, which excludes most humans. Nevertheless, they are quite capable of influencing their environment. When souls with exceptional spiritual energy train their bodies, they reach the level of Shinigami. The most talented of them become affiliated with various organizations like the Thirteen Court Guard Companies and the Stealth Force. Sometimes, a Shinigami is born among the the residents of the Rukongai, and in some rare cases from Seireitei. Shinigami are the personification of death. Their job is to send spirits to the Soul Society (the afterlife) in order to maintain a balance of souls between it and the material world, and to exorcise evil spirits.

Having begun as Shinigami, the Vizards appear entirely Human, and barring those times where they are using their powers are indistinguishable from ordinary Shinigami. To gain access to their Hollow powers, a Vizard dons a Hollow mask. This allows them to augment their Shinigami abilities with Hollow powers, as well as giving them access to some of the more generic Hollow abilities. Their spiritual pressure likewise changes into a mix of Hollow and Shinigami. This process of donning one's Hollow mask is referred to as Hollowfication.

Hollows are former Pluses that lose their hearts to despair or remain in the real world for too long. In the anime, nonhuman spirits can also transform into Hollows. Any spirit that is not guided to the Soul Society by a Shinigami via Soul Burial may eventually descend into a Hollow.

There has always been a small number of incomplete Arrancar in existence, but after Aizen brought the Hōgyoku them, they were able to completely transform into true Arrancar. An average Hollow that removes their mask would have to at least be Menos-level or stronger. Few Hollows have the potential to remove their masks naturally, and even if they manage to do so, the change in power may not be significant. These hollows went through the Shinigamification

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 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.