This article contains spoilers from Bleach: Thousand Year Blood War, currently streaming on Hulu.All the events of Bleach are set within one of three domains: Seireitei, Hueco Mundo, and the Human Realm, and a select few can travel between these worlds. At face value, the Shinigami are the good guys, Hollows are bad, and the Quincy are, well, mostly a mystery.
So far in the Thousand Year Blood War, it looks like several Gotei 13 Divisions are on high alert regarding hoards of missing Hollows, which seems strange considering that these sinister supernatural beasts are treated as the enemy. However, it is clear from the look on Mayuri's face, that this is far from a blessing in disguise! Seeing that the Shinigami spend most of their energy fighting these monsters themselves, why are they suddenly so concerned about the disappearing Hollows?
The Cycle of Souls
Essentially, the entire plot of Bleach revolves around the concept of reincarnation while questioning what happens if the natural order is disrupted. According to the novel; "Can't Fear Your Own World," the world was once a singular realm overrun with Hollows until the Soul King suddenly emerged. Dissatisfied with the blatant abuse of power, the Soul King exterminated the harmful Hollows, leaving the innocents to continue life without harassment. As with any natural order, removing the hunters would disrupt the food chain significantly, and this case was no different.
In an attempt to regulate the system, five mighty entities (who became the five Noble Shinigami Families) united to imprison the Soul King inside a crystal, slow his mass killings, and siphon off his energy. They were thus able to separate the world into the Human Realm, Seireitei (aka Soul Society) where the souls were to reside, and Hueco Mundo for the Hollows. This transformation became the origin story of the cycle of souls, whereby the flow of spirits follows similar principles to Newton's Law of Conservation of Energy (in theory, anyway.)
This new world lacked the ability to generate more souls, so great care is needed to maintain a healthy balance of energy on all three flanks. Humans who die can enter Soul Society, and when a new child is born, a spirit is sent back, and thus, the cycle continues. However, lost souls eventually turn into Hollows and tend to drift over into Hueco Mundo instead (although, unfortunately, some stay among the living to cause further chaos.) In a perfect world, this design would work, however, not everyone is on the same page regarding ideologies.
The Shinigami
The Soul Reapers are responsible for maintaining order in Seireitei as well as protecting the world from wayward Hollows that threaten the lives of others. They also work to safeguard the transition of human souls (called pluses) through to the afterlife using a ritual known as Konsō or "Soul Funeral." Depending on the dearly departed's level of Reiryoku (or spirit energy) they will subsequently relocate to the Shinigami-producing Seireitei or the disadvantaged Rukongai region in Soul Society. Hollows who find themselves at the back end of a Zanpakuto will have their sins washed away, and their cleansed souls transform back into pluses and are subsequently welcomed into Soul Society.
The Hollows
As with any vengeful spirit, those burdened with any unfinished business struggle to pass over and are stuck wallowing in their own despair until their Soul Chain is eventually severed and they become Hollows. To grow stronger, a Hollow will absorb other spirits and soak in their power, which temporarily removes that energy from the soul cycle. While Hollows appear to have a one-track mind and an unmanageable appetite, once reaching the Vasto Lorde status, that eternal hunger seems to subside. Devoured souls are not permanently exterminated but essentially reside within the monstrous form and would theoretically all be released back into Soul Society if a Hollow Cleansing is successfully performed.
The Quincy
Much like the Soul Reapers, the Quincy see it as their duty to protect the innocent from rampant Hollows, but without direct access to Seireitei, their process of elimination is significantly different. Instead of acting as a Grim Reaper of sorts, the Quincy Clan aim to destroy rather than relocate; wiping each Hollow they fight out of existence completely as opposed to redistributing their energy. They also heavily rely on either collecting Reishi or absorbing Reiryoku to fuel their Spirit Weapons, which arguably makes them the most disruptive in the natural order of things.
A Difference of Opinions
It might seem crystal clear who the good guys are in this whole scenario, however, first impressions can be deceiving. The Quincy are so adamant in contributing towards the fight against Hollows because they do not agree with how selective the Shinigami's process is. Even though Seireitei has a substantial population of capable warriors, they do not send out Soul Reapers for every single Hollow attack and rather opt to save their resources for the more severe cases. "Sacrifice a few to save the many," so to speak, whereas the Quincy wished to protect everyone, regardless; and the two parties subsequently bashed heads.
The Shinigami were very reluctant to accept the Quincy's help with Hollow removals because of how permanent their solution is. For the Conservation of Energy to remain intact, energy cannot be destroyed, and therefore every kill that a Quincy makes subsequently causes disequilibration. A Hollow that is refused mercy cannot enter Soul Society, however, humans are still born with a reincarnated spirit (of which there is only a finite number available), making the flow rather one-sided. And remember, a Hollow contains the souls of all those it has devoured, which are also wiped from existence once a Quincy lands a fatal blow.
The Repercussions
Many moons ago, when things started to get out of hand, the Soul Reapers turned their Zanpakuto on the Quincy and caused a mass extinction of their (essentially human) forces, for the sake of the greater good. However, there is a valid reason for the Shinigami's severe regulations regarding the cycle of souls, as they are well aware of the disastrous effects of a toppled system. If one of the domains becomes overburdened with souls, it will virtually overflow into the others, causing the Human and Spiritual Realms to become one. Then, it would not be long before the world reverted back into the Pre-Soul King dystopian state.
Yhwach, however, does not believe that the worlds were not meant to be separated in the first place (who gave the Shinigami the authority to make such drastic decisions?) According to his theory, if the living and dead walked among one another, there would be little differentiation between the two, which would subsequently eliminate all "fear of death." Yhwach believes his father, The Soul King (yes, that's right!) should have never intervened in the first place and plans to rectify the matter with his Invisible Empire, The Wandenreich, by any means necessary.
Source: reddit/@UrielSans