And so, after many, many more years passed, the Royal Clan — the Kazama family gradually declined due to their inability to obtain fresh bloodlines.
Because of the decline of the Kazama royal family, Zero Division also began to face a shortage of talent and had no choice but to request capable individuals from Soul Society.
Captain-Commander Yamamoto Genryūsai and Unohana Retsu had the strength and qualifications, but they were too experienced and proud—preferring to be the head of a chicken rather than the tail of a phoenix. Naturally, they had no interest in going to the heavens to serve as someone else's subordinates.
Kyōraku Shunsui and Ukitake Jūshirō also looked promising, but Yamamoto Genryūsai refused to release them. That was understandable, since they were essentially his direct disciples.
Among the remaining captains, those born from noble families—represented by Kuchiki Ginrei, captain of the Sixth Division—were either too old or too young and not suitable candidates.
Meanwhile, those from the Rukongai, represented by Hirako Shinji, captain of the Fifth Division, were either too unserious, fake-serious, or simply not strong enough.
These people could only really be kept in line by Yamamoto Genryūsai. No one else could control them. If they were sent up to the Royal Guard, they might cause huge trouble—worst case scenario, they might even accidentally kill the Soul King, and then everyone would be finished.
In the end, with no other choice, Zero Division had to pick the best from the worst. The honest and relatively strong Kirio Hikifune, captain of the Twelfth Division, was sent up to the Royal Guard.
As for the Four Great Noble Families, during the early years of Soul Society they had everything they wanted. All nobles and even commoner Shinigami followed their lead.
But then a hot-headed figure named Yhwach suddenly appeared. He formed a Quincy organization that swept through the Human World and Hueco Mundo undefeated, and then launched a full invasion of Soul Society.
The four noble families, who had spent years scheming against one another, had long since lost the combat power their ancestors once possessed. They were beaten badly by the Quincy forces, suffering heavy casualties and fleeing in panic—laying the groundwork for their later decline.
Just as Soul Society was on the verge of falling to Yhwach, Yamamoto Genryūsai Shigekuni rose to prominence. He gathered powerful warriors from across Soul Society and formed an invincible assassin group.
After a long and brutal war, this assassin group—led by Yamamoto—completely annihilated Yhwach's Quincy army. Yamamoto himself personally defeated Yhwach.
Perhaps because of the saying "once the cunning rabbit dies, the hunting dogs are cooked," or perhaps because Yhwach learned the trick of faking death, in any case, Yamamoto did not completely kill him at the time.
After the war, under mediation by the Kazama royal family and Zero Division, the four great nobles reached an agreement with Yamamoto's assassin group and established the Gotei Thirteen.
Naturally, Yamamoto Genryūsai became both Captain of the First Division and Captain-Commander. The remaining twelve captain seats were theoretically split evenly between Seireitei nobles and Rukongai commoners.
Noble-side divisions (2, 6, 7, 8, 10, 12)
Second Division — Onmitsukidō
The captain traditionally comes from the Shihōin family, one of the Four Great Noble Houses.
110 years ago the captain was Shihōin Yoruichi. After the "Captain and Vice-Captain Defection Incident," the vice-captain Soi Fon succeeded her.
Sixth Division — Noble-exclusive division
All members are nobles, and the captain has always come from the model noble family: the Kuchiki Clan.
110 years ago the captain was Kuchiki Ginrei. After the defection incident, he gradually retired due to age and intended for his grandson Kuchiki Byakuya to succeed him.
Seventh Division — Inner Court Guard
Earlier captains came from the noble Ryōdoji family. However, after their research into Hollow levitation became too dangerous, the entire family was expelled by the Central 46 and wandered the Dangai for over a thousand years.
110 years ago the captain was Love Aikawa from Rukongai. After the defection incident, the current captain became Komamura Sajin.
Eighth Division — Intelligence Division
After the previous captain retired, the position was taken by Kyōraku Shunsui, a disciple of Yamamoto and a high-ranking noble.
Tenth Division — Patrol and Security Division
Traditionally led by the Shiba Clan, one of the Four Great Noble Houses.
110 years ago the captain was Shiba Kazuo, head of the Shiba main family. Around the time of the defection incident he died in the line of duty, and his brother Shiba Isshin, head of a branch family, succeeded him.
Twelfth Division — Technology Development Division
Originally led by members of the Tsunayashiro family, another noble house.
After the scandal involving Tsunayashiro Tokinada murdering his wife and plotting against colleagues, the family lost the seat.
110 years ago, after Captain Kirio Hikifune joined Zero Division, the captain position became temporarily vacant. The declining noble Kisuke Urahara, then the Third Seat of the Second Division, stepped in.
After the defection incident, Mayuri Kurotsuchi, vice director of the Research and Development Bureau, became captain.
Commoner-side divisions (3, 4, 5, 9, 11, 13)
Third Division — Support Division
110 years ago the captain was Rōjūrō Ōtoribashi.
After the defection incident, the role was temporarily overseen by Aizen Sōsuke, who had become captain of the Fifth Division. Many years later, Aizen recommended his vice-captain Ichimaru Gin as the new captain.
Fourth Division — Medical Division
Originally vacant until Unohana Retsu transferred there. She has served as captain ever since.
Fifth Division — Rescue Division
110 years ago the captain was Hirako Shinji, representing captains from Rukongai.
After the defection incident, vice-captain Aizen Sōsuke succeeded him as captain.
Ninth Division — Prison Guard Division
110 years ago the captain was Kensei Muguruma, nicknamed the "unit of measurement."
After the defection incident, the sole survivor of the mysterious "Night Tragedy," Kaname Tōsen (formerly Fifth Seat of the Ninth Division), became captain.
Eleventh Division — Combat Division
The captain holds the title Kenpachi.
The first Kenpachi was Unohana Retsu, who later transferred to the Fourth Division for unknown reasons, leaving the title to future captains.
110 years ago the captain was the 10th Kenpachi, Kenpachi Kiganjō. After the defection incident, he was defeated by Zaraki Kenpachi.
Thirteenth Division — Purification Division
After the previous captain retired, Ukitake Jūshirō, a disciple of Yamamoto and a lower noble, became captain and remains so.
Thus, judging from the current state of the Gotei Thirteen, the number of noble captains has been steadily decreasing, while more captains now come from Rukongai.
In other words, the number of captains loyal—or at least outwardly loyal—to Yamamoto Genryūsai has been increasing, to the point that even the five great noble families must defer to him.
As time passed, the five noble families increasingly felt their power and survival space shrinking because of Yamamoto's influence. Secretly, they contacted Zero Division.
Recognizing that Yamamoto's power was becoming a growing threat, Zero Division agreed to the nobles' request. Using the remaining fragments of the Soul King, they secretly cultivated a supernova, naming him Aizen Sōsuke.
The mission they gave Aizen was simple: become a troublemaker, stirring chaos within the Gotei Thirteen.
Aizen completed the mission perfectly.
He secretly developed his own rebellion forces, while also researching ways to break the limitations of Shinigami in order to escape Zero Division's control.
Eventually, he created the Hōgyoku, breaking the boundary of Shinigami evolution and becoming the first super Shinigami to evolve into a "butterfly."
And as a born leader, Aizen naturally refused to remain under Zero Division's command.
Thus, he officially led his two trusted subordinates and launched an attack on the Soul King Palace.