The Order of the White Knights, is an ancient organization of the eldest Shinigamis and was formed before the Gotei 13 was established.
The Order of the White Knights were formed before Gotei 13 was established, each members wear a white haori as their uniform, it is known that Genryūsai Shigekuni Yamamoto, who later became the Captain-Commander of the Gotei 13, patterned the Captain's uniform after the uniform of the White Knights. Each of its members are Captain-class warriors, some even have greater power than Captain-class warriors. Unlike the Espada, the Order has no sort of hierarchy whatsoever.
The group was founded when four shinigamis who had grown together in a place filled with corrupt shinigamis and evil hollows who kills many innocent souls, so to make a change, the four who were known to be Karakura Gozei, Shigekuni Yamamoto, Kyoza Shinke, and Mikuni Sobuhara trained together to become powerful and formed the group, many years later, more shinigamis joined the group. Almost for a century, they were considered as the most powerful group, but all things changed when one of their member became corrupt and power hungry, he was known to be Karakura Gozei, the Order had no choice but to kill him, but the remaining three founders cannot bear to kill him, so instead of killing him, they sealed him in the real world, to a place where in many years later will be known as Karakura Town, but the reason of who named the town and how he did it was left as a mystery.
There are an unknown number of members the Order, while it is well known that each one of them are Captain-class and some of them are more than Captain-class. Some of the well-known members include the previous Captain-Commander of Gotei 13, Shigekuni Yamamoto and the Shinigami whose name was used to name Karakura Town, Karakura Gozei.