Shinigami “Soul Reapers”
Shinigami are a race of spiritual beings that act as enforcers and psychopomps who all share similar supernatural powers. They are souls with a high level of innate Spiritual Energy, recruited from the ranks of the residents and nobility of the Soul Society. Like all spirits, they cannot be detected by normal humans. Shinigami use their Zanpakutō, supernatural swords that are the manifestation of their owners' power, to perform soul burials on Pluses. Shinigami also use their Zanpakutō as well as magic known as Kidō to fight Hollows and Bounts.
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.
Shinigami operations are based out of the Soul Society, where departed human souls reside. Travel between the human world and Soul Society is extremely limited and monitored, but some Shinigami are stationed in the human world to carry out their duties and therefore must often travel between the two.
Most Shinigami appear like Humans, a notable difference being their mostly old-fashioned garments by 20th-21th century standards and their Zanpakutō which is in most cases tied to their obi sash. There are, however, a few Shinigami who are less Human-like in appearance. For example it seems that between Shinigami there are much greater variations in body-sizes than between Humans, as there are real giant-like entities (e.g. the four guards of the four gates to Seireitei) and also ones with extremely small stature among them (e.g. Yachiru Kusajishi). This variation seems to occur not just between Shinigami, but also between regular dwellers of Soul Society, too.
Variations do not just occur in size but in overall appearance, too, the most notable example being Captain Sajin Komamura, who appears like an anthropomorphic canine.