Quincy (Seireitou)

The Quincy (滅却師 (クインシー), Kuinshī; Japanese for "Monk of Destruction") are Humans classified as Psysises that possessed the power to perceive the world of spirits and connect with the beings of that world, subsequently allowing them to detect the existence of Hollows. They are the polar opposite of Shinigami, in the same manner as the ancients depicted yin and yang, which has caused multiple interactions of hostility between them throughout centuries.

History
Originally, Quincy were known as humans that could serve as a bridge between the physical and the spiritual worlds. They were known by many names, such as priests and exorcists, but in ancient lore, were known solely as shamans. The nature of their reiryoku was different than other Psysises, in that it sought to connect with spiritual beings in the form of allowing Quincy to connect their internal spiritual power with reishi; that which composes the beings of the spiritual world. The original purpose of Quincy was to connect humans with the spiritual world, so that they would not fear that which they could not normally perceive by their own power, and this would lead to an everlasting peace.

The Shaman, a being known only to few that study the ancient Quincy lore, is regarded as the one who first possessed this power and utilized it as a means of connecting humans with the spiritual world. During this time, the Quincy were known under a different name, the "Unifiers" (though written in an ancient dead language that no longer exists), referring to their purpose of serving as a bridge between worlds and the mediators for peace and understanding.

However, while this was the intention, the Quincy chose instead to connect their spiritual energy with free-flowing reishi and impose their will over it, so as to construct a means of battling Hollows and other beings they deemed threats. The will to create violence and spread dissension between the physical and the spiritual worlds was what caused the pure-intentioned ability of Quincy to become tainted with hatred and selfishness, turning it into a power that destroys souls rather than purifies them. This is the basis for modern-day Quincy.

Behind the Scenes
The basis for the Quincy's original purpose was inspired by the concept of Ninshū