What's Next?

The Quincy

There's a limit to how much a human body and mind can withstand in terms of channelling spiritual energy from their own soul through their own body - because shinigami are spiritual bodies, they bypass this constraint (and their spiritual energy is also different in nature, connected to the afterlife of soul society rather than the living world), but spiritual energy strains and damages living bodies (see, Uryu using his powers to the point it cuts his arms - his are concentrated and focused and practiced, Orihime and Chad passing out after using their powers for the first time). One of the key features of the Quincy is in mitigating this constraint by using spiritual energy from outside themselves. It still takes spiritual power to draw in other spiritual energy, and much more spiritual power to control and contain that energy to do what you want, but it massively multiplies the power you're able to use. (There's still the danger of too much - see, Uryu's arms getting cut up with overuse, the massive amount of energy he was able to pull in with the glove burning through his own abilities when not diffused, the cero burning out his powers) The other key feature of the Quincy is the use of a range of tools and components to enhance their ability to gather and control spiritual energy. The development of increasingly advanced talismans and tools also included enabled people without much spiritual power to  be able to defend themselvesas Quincy with Quincy techniques. There are other traditions that have independently discovered these approaches (and more individuals and traditions that simply draw on their own spiritual energy), but the Quincy were the most widespread.

The way Orihime's powers work is fundamently the same as the Quincy, just developed independently. Orihime and Chad both have a natural aptitude for drawing spiritual power from around them, which is why Ichigo's overflowing spiritual power affected them. Orihime's powers center around her hairpins, like Quincy bows center around a personal spiritual artifact. Whether there was a good luck charm on it when it was purchase or if the love and affection around the hairpin attracted positive spirits or both is unknown. But with Ichigo's spiritual energy there to use for growth, Orihime's own spiritual power and soul gave the spiritual energy around the hairpin consciousness and personality, and they were able to develop into their power. Because they draw on spiritual energy from around, Orihime is able to get a lot more powerful - though that control relies on her own mental toughness and also the strength of her imagination and will.

Chad's powers are mostly him channeling his own spiritual energy into and through his own body, (though in environments rich in spiritual energy he does draw from the environment - which is why we see that effect around his arm in soul society) which puts him under a lot more strain - his mental toughness and conviction and physical fortitude are the reason he can do what he does - he's exceptionally powerful, and because he already has an aptitude for absorbing spiritual energy, he'd very easily be able to learn Quincy methods. Tatsuki, after her training with shinigami and others during the war, is in a similar boat to Chad, just with a little less practice - she's channeling her own energy into physical powers.

In the course of Chad and Tatsuki training in the context of shinigami, I think they'd learn with the shinigami like Rukia and Renji around them learning that living human spiritual power destroys hollows by default - because it's not specifically shinigami spiritual energy tied to the afterlife, it doesn't purify, shinigami are doing something very specific. Living energy has a natural opposition of the dead. Unless Chad and Tatsuki are using some shinigami specific tool, they're annihilating hollows, and Orihime is also annihilating hollows. Humans with no connection to soul society have no way of knowing what is happening to hollows they destroy, annihilation or returning to some sort of afterlife. There may be some individuals who have historically developed purification abilities, but it's basically impossible to determine from records, because humans don't know what happen to hollows they destroy.

The whole 'Quincies insist on annihilating hollows while shinigami purify them and maintain the cycle' is a convenient (for shinigami) distortion.   The shinigami basically told the pre-genocide Quincies the same thing they told Uryu's grandfather, 'don't interfere with our work' and to stop killing hollows because it would throw off the balance, and Quincies insisted on continuing to kill hollows because hollows killed loved ones and people around them, and waiting for shinigami and not killing hollows meant letting more people die. As Uryu says, shinigami are slow operating from the peace of soul society, that's something that was well known, Uryu's grandfather cited it as a reason for Quincy to work with shinigami, because Quincy in the living world respond more quickly.

And Uryu's grandfather is a case where that delay was intentional, but we see that generally - Rukia is out of contact for months and nothing happens. Thanks to Uryu's hollow bait, hollows swarmed Karakura, and then there was a menos grande, and there was no shinigami response. Shinigami are by their role meant to be disconnected from human emotions and, importantly, to be part of the process of human death - of course they can't see every human death as a tragedy. If a hollow kills/consumes more humans, they're just going to soul society when the shinigami comes to purify the hollow and spirits. (And for shinigami specifically, the 'peace' of the Gotei 13 is not necessarily the situation in the rest of soul society, which many of them all too quickly forget) For living people, that's their family, friends, and neighbors dying. There may have been some Quincy with a strong opinion that Hollows should be annihilated, after being presented with the alternative of waiting around for them to be purified, but in general it was a case of it being easier for the shinigami to represent this as a mere ideological difference than acknowledging it was a matter of life and death.

Quincy history - some of the earliest recorded instances recognizable as what would become Quincy traditions are among travelers in Europe, some Romani, who gathered and shared knowledge across different communities, accumulating a range of practices. As those humans with spiritual powers and/or their own traditions to fend off evil spirits often ended up on the fringes of society, outsider travelers had ample opportunity for knowledge exchange with other spiritual experts. The Quincy started to develop a significantly powerful tradition post Columbian exchange. The thinnest part of the spiritual barrier between the world of the living and Soul Society is in the Pacific near Japan, Hueco Mundo on the opposite side of the globe, in the Atlantic off of Argentina. Argentina and other parts of South America therefore have significant traditions to fight hollows, particularly in using natural spiritual energy from their surroundings effectively, and had their own artifacts and talismans, which are also recognizable as early precursors to Quincy traditions. Through individual and small scale exchanges of knowledge, exceptional tools and teachings were developed into what is known now as the Quincy tradition, taught and shared with those with spiritual power, especially travellers. (Not to ignore the horrors of colonialism going on or say that wasn't involving in any of the interactions, but) 

In what would become Germany, scholars with an interest in science and magic (think Newton's interest in alchemy) began documenting these tools and techniques. In many (Christian or colonized by Christians) places, a cross was a 'safe' symbol to be associated with casting out demons without being accused of evil, some adopting it to feign conversion, some traveling Quincy posing as priests conducting exorcisms, or were sincere Christian believers who interpreted the techniques in the context of their faith. (But there would also be plenty of Quincy using symbols of their own religious traditions or Quincy specific symbols like the five pointed star)  

With the efficacy of the new techniques, they began to be more perceived by soul society over the decades as teachings spread. (The shinigami, on their own timescales, would consider decades rapid growth) The quote-unquote proto-Quincy in Japan were a significant group and eventually gained from this transmission of knowledge. These Quincy were the first to receive communication from Soul Society about the threat to the balance. 

One shinigami was documented as giving their power to a Quincy in dire straits, allowing the Quincy to purify hollows. They were both killed for their trouble. No alternatives to protect people from hollows were offered by the Gotei 13, except to wait for the shinigami to do their work.

Being closest to the Shinigami place of strength, the Quincies in Japan were hit hardest  by the subsequent genocide and there were no recorded survivors. Worldwide, other groups of humans or notably powerful infividuals with spiritual powers who were not Quincy, got targeted by the genocide. (They weren't exactly stopping and asking, excuse me, do you identify as a Quincy, they were committing a genocide) The Quincy being so widespread and the shinigami only having bothered to reach them individually when going to kill them, many Quincy had no context for the shinigami's opposition, or in some more far flung cases, little to no context for the shinigami at all. In addition to killing people, shinigami destroyed any Quincy tools or records they found

The Quincy researchers in what would become Germany were some of the few that were able to save tools and extensive records through a magically shielded space. Those were preserved and copied by Quincy, tools reproduced and redeveloped, maintained from the genocide in the 19th century into the 20th century. This is why most Quincy terminology we see is German, because they trace from those records. Other places have some of their own terms, some loan words, depending.

Prior to WWII, a group of various German Quincies, some Jewish, some Romani, some not, saw the writing on the wall in Germany and took copies of Quincy records and fled the Nazis to Mexico, while others fled other places. The group fleeing to Mexico included Uryu's great-grandmother. From Mexico, some Quincies travelled south in efforts to reincorporate the records surviving from Quincy communities in central and South America, and some central and South American Quincies came north to Mexico to join Quincy community and share knowledge. In the present the largest and most extensive collection of records is in a magically shielded archive in Mexico. 

Uryu's great-grandmother met a Japanese man - actually specifically Okinawan - in Mexico, who had left and found work ahead of the war as well. They married, he joined the work of supporting the Quincy, and they had a child who would be Uryu's grandfather. Some years after the war's end they went to Japan, possibly with others, with Uryu's grandmother hoping to find any surviving Quincy and share and preserve the Quincy knowledge elsewhere. They were somewhat successful, with a preserve of knowledge entrusted to Uryu's grandfather - again, almost entirely in German - but it was only ever a relatively small group of people established.

One of the other families who came in would be Uryu's mother's family. By the time Uryu was alive, most others had been killed by hollows and been taken for Mayuri's vivisection, or had left. His mother was more invested in the Quincy and protecting people from hollows - protecting everyone she could, than Ryuken, who was already one foot out of this tradition and feared the shinigami and feared in any way being seen as 'other' in a negative way. Uryu's mother died fighting hollows when Uryu was maybe five or six, protecting Uryu's paternal grandmother, who was probably protecting other people herself (Ishidas keep ending up with people who want to protect everyone) - which probably means Uryu's mother's spirit was also tortured and vivisected by Mayuri. His wife's death killed any tolerance Ryuken had for the Quincy side of things and generally shut him down as a person, such that Uryu was raised largely by his grandfather from 5~9, until his grandfather was killed by hollows due to intentional shinigami negligence.