Saisho no Gādian

The Saisho no Gādian (最初のガーディアン, First Guardians) are known to be the first four Shinigami to have ever existed. Though much of the information concerning them is lost, it is known that they founded the earliest version of the Gotei 10, and that what is believed to be their original Zanpakuto still stand at the four gates of the Seireitei, whose spirits act as the gate guardians. To those Shinigami who still know of Aspects, they are aware that the Saisho no Gādian were direct descendants of four Psychopomps. The fate of the Saisho no Gādian is still largely unknown.

History
Many hundreds of years ago, human souls with notable reiatsu began appearing due to humans and Aspects procreating. The offspring of these "halfspects", originally sent to The Aether, began to get sent simply to the Soul Society due to the Aspect in their blood getting to diluted to justify bringing them into the Aether. Four of these powerful humans decided to try and organize other powered humans, and make a deal with the Aspects to help in psychopomp duties. This deal was the precursor to the First Aspect Treaty, though the four 'Shinigami' wanted to be partners with the Aspects, instead of getting rid of them.

Though much of the history leading up to the First Aspect Treaty has been lost, it is known that each member of the Saisho no Gādian invented and coined the terms for one of each of the four Shinigami arts, Hoho, Kido, Hakuda, and Zanjutsu. It is also known that, in order to prove themselves worthy to the Aspects, the four guards had to seek out and capture four rogue Aspects who had broken several laws, and seal them away into special blades given to them by Nature. These blades still hold the rogue Aspects, and now serve as the gate guards for the Seireitei.

After the First Treaty, the organization of the Gotei came to resemble the modern version, with four divisions headed by the Saisho no Gādian, instead of thirteen, and more were added over the years as more Shinigami joined. However, after the First Shingami Civil War, the Saisho no Gādian disappeared and their fate is still unknown. Nothing is left of the four except what appeared to be their Zanpakuto, left planted in the ground in front of each of the Seireitei's four gates. When approached for the first time, the conquered Aspects revealed themselves, and the reason they were left there, forced into obedience by Nature. Though they continue to serve as the gate guards, modern Shinigami believe them to simply be the Saisho no Gādian's Zanpakuto spirits, somehow left behind.