SUMMARY
-
Aizen's blurring of the boundaries between Shinigami and Hollow led to the creation of more Arrancar.
-
BG9's visual and thematic similarities to Shawlong, along with data collection, have given rise to theories about their relationship.
-
There is some uncertainty as to how an Arrancar like Shawlong became a Quincy like BG9 due to the difficulty of purification and rebirth.
the bleach The universe is expansive and complex, with various confirmations of the main kingdoms introduced over the course of the series made possible in part by Sоsuke Aizen's (and to some extent, of Kisuke Urahara) on the border between Shinigami and Hollow. Using the Hо̄gyoku he took from Rukia's soul at the conclusion of the Soul Society arc, Aizen goes on to create more Arrancar, Hollows who gain Shinigami powers by tearing off part of their mask. .
In the A thousand year blood war arc, we learn that Yhwach's Wandenreich has advanced into Hueco Mundo, bringing many Arrancar into their ranks after Yhwach's defeat of de facto ruler Tier Harribel. This use of Arrancar in fighting Shinigami is what inspired fans to draw parallels between the Quincy robot BG9, and the Arrancar Shawlong Koufang. Is there any weight to the idea that BG9 is Shawlong?

Connected
Bleach: Arrancar Evolution, Explained
The evolution of the creatures known as Arrancar is much more complex than you might think.
Who is Shawlong?
One of Grimjoww Jaegerjaquez's few allies
The Undecimo (11th) Arrancar in Aizen's army, Shawlong Koufang was one of the five Fracción of Grimmjow Jaegerjacquez. His number indicates that he was the oldest Numero in Aizen's Arrancar army before his death in Karakura at the hands of 10th Captain Tо̄shirо̄ Hitsugaya. At one point, Shawlong was a Gillian with a personality that evolved into Adjuchas-Class Hollow and led a small group of other Menos Grandes including Di Roy Rinker, Octava Espada's brother Szayelaporro Yylfordt Granz, Edrad Liones, Nakeem Grindina and two different. Nameless Adjuchas, wandering around eating the Hollows they come across, until one day a little Adjuchas. This incident led the group to be eaten, Shawlong announced the small Adjuchas, Grimmjow Jaegerjacquez, the new leader.
The group had the desire to evolve into Vasto Lordes; however, they do not realize that the Hollow's development stops when consumed by others. Realizing this, they gave up and gave their bodies to Grimmjow to eat to be part of his evolution, becoming the five Fracción. Shortly after Ulquiorra Cifer and Yammy Llargo first appeared in Karakura, Ulquiorra sent Shawlong and his friends to kill those with high spiritual pressure in the area, thus participating in the battle with Hitsugaya. At first, the Arrancar had the upper hand in their battle, but when the Shinigami officers were allowed to unleash their full power in the Human World, they were quickly defeated. Shawlong was adept at using Pesquisa, translated as “Investigative Nerves”, a technique that allowed him to send out pulses that then interacted with the Reiatsu sources in the area and determined their power.
Why some people think BG9 could be Shawlong
The evidence is pretty damning
In the A thousand year blood war arc, we see the Quincy robot BG9 fight several Shinigami Captains, with his fight against 2nd Squad Captain Soi-Fon being the Quincy's standout moment. During the second phase of the A thousand year blood war arc, BG9 uses needle-like extensions from its body to poke enemies and extract all kinds of data from its victims. This happened to 2nd Squad Marechiyo's younger brother Lieutenant Omaeda, who BG9 knew was related to him and reading many other Reiatsu in the immediate area, and even after he smeared Soi -Fon though, BG9 gained an understanding of his abilities.
This collection of data has inspired the theory that BG9's name, K, means “Knowledge,” but we may never know. Like Shawlong, BG9's head is shaped like a mask, with vertical lines on its face, and interestingly, BG9's needle extensions are reminiscent of Shawlong's long claws in the Resurrection. Combining the similarities between Shawlong when he was Adjuchas and BG9 with the relationship between the former's control of Pesquisa and the latter's “data collection fish”, it is clear that there is switching between the two, including the fact that the same Japanese voice actor, Hideyuki Tanaka, is both characters.
What reduces the probability of theory
Hitsugaya didn't clean Shawlong?
We learn at the beginning of bleach but Shinigami purify Hollows by defeating them with their Zanpakutо̄, allowing them to return to the cycle of reincarnation. However, things escalate with stronger Hollows and Arrancar. At some point a soul becomes too powerful to return to the cycle of reincarnation without affecting the balance of the realm. This is why when Shinigami Captains die, they are sent to Hell through a special kind of Konsо̄.
In the BLEACH: No breath from hell one-shot, Unohana, Ukitake and Yamamoto are sent to Hell where they receive epithets based on their lives as Shinigami. The important person here is Szayelaporro, the only character we actually see coming out of Hell, which means that after killing Kurotsuchi, the Hollow did not return to Soul Society but was sent directly to Hell. On the other hand, considering that Arrancar are generally part of Shinigami, the idea that they would go to Soul Society after being killed by Shinigami is questionable.
When a Shinigami is killed by someone else, they become a part of their surroundings as they are simply reborn as Reishi. The only way the Quincies can turn Shawlong into BG9 is if he didn't actually die in the battle with Hitsugaya (highly unlikely), or if the process of purification and rebirth is something that interferes. the Hollows. after being killed by the Shinigami.
If he is too weak to be sent to Hell, and too much of a hybrid to be completely purged, he becomes part of the Reishi around him, perhaps due to some mechanism of manipulation by the Quincies. of Reishi, he can become a part. of the Sternritter in the same way that Kaien Shiba became part of Metastacia, who then became part of Aaroniero Aaruruerie. It's an interesting idea supported by the Arrancar documented Quincies, but it's not clear how Shawlong turned into BG9 despite the visual and thematic similarities.

Bleach: A Thousand Years of Blood War