Bleach Qunicy:The Quincy (滅却師 (クインシー), Kuinshī; lit. "Monk of Destruction" in modern terms, they were Human mediums, having the ability to detect the existence of Hollows. It all started when they began to training to confront them in order to defeat Hollows, the same as Shinigami do. Most of the Quincy were destroyed by the Shinigami over 200 years ago in order to prevent the destruction of the world.
Quincy are spiritually aware humans able to absorb and manipulate Reishi into spiritual weapons.
Reishi Absorption: Although Shingiami and Quincy both use Reiryoku when fighting, they both have a totally opposite fighting style. Shinigami use the spiritual power created from their soul in conjunction with a Zanpakutō to fight against the Hollows. You can say they fight using the power within. On the contrary, a Quincy uses the Reishi in the atmosphere, and combines it with his/her Reiryoku to create weapons. They fight using the power outside. They have an easier time collecting this energy in environments with higher concentrations of spiritual particles such as in Soul Society or Hueco Mundo. To a sufficiently powerful Quincy, there is no difference between spiritual particles and spiritual energy as a source of power.
Spiritual Awareness: As a spiritual being, the Quincy can sense Hollows and other spiritual beings at a significant distance or tell in what direction they are in.
The Quincy were scattered all over the world at one time. They were relatives of magic hunters who specialized in the war against Hollows.
Since the origin of the Quincy, many years passed within Soul Society trying to appeal to the Quincy that the handling of Hollows should be left to the Shinigami. The Quincy refused to accept the situation. As time went on, the number of Quincy increased and the cycle of souls became more unstable, forcing the Shinigami to reluctantly wipe out the Quincy in order to maintain the balance of the world.
The Quincy line was closely monitored over the decades, as their numbers continued to dwindle. Later the Quincy advanced the idea that since they were already among the living and still more numerous at the time, they could act as "first responders" to Hollow attacks; protecting Humans until the less-numerous Shinigami could send a representative to finish the Hollow off. Embittered by the recent war, the Shinigami refused to participate, opting instead to allocate their limited resources to reacting to the most dangerous Hollows and current Hollow attacks. This caused great enmity towards Shinigami by Uryū Ishida, Ichigo's classmate and a Quincy, whose grandfather, the last advocate of the Quincy plan, had been slain by a team of Hollows before the Shinigami could arrive.
Danger to the Balance
Shinigami govern the souls between the Human World and Soul Society, which maintains a balance that if not kept would result in the mutual destruction of both worlds. All souls, including Hollows, are subject to the Shinigami governance of maintaining the balance. Quincy completely extinguish Hollows. Thus the souls that come to the Human World do not return to Soul Society and therefore the souls would increase only on the side of the Human World. Meaning, the Human World becomes heavy in souls causing Soul Society to spill over into it; mixing life and death. The ultimate result would be the end of the world.