This article contains spoilers from Thousand-Year Blood War and Can’t Fear Your Own World, not yet revealed in the Anime.
The Soul King is essentially the most important character in Bleach, as he is responsible for maintaining the balance between the human realm, Sereitei, and Hueco Mundo. Fans are first made aware of his existence when Aizen reveals that the destruction of the Soul King is a crucial part of his grand scheme, but the corrupt Captain does not go into great detail. Not much more has been mentioned about this omnipotent entity since, however, Yhwach revives viewers' interests in the Soul King during the Bleach: Thousand-Year Blood War Arc.
The Soul King has the power of premonition but is above intervening in the lives of mere mortals. He is focused solely on maintaining a healthy flow of souls between the realms, similar to that of Charon, the Boat Man in Greek mythology, but substantially more powerful. For some undetermined reason, the Soul King is without arms and a heart, as each has been severed from his body, only to become sentient themselves. Each body part has a different purpose and two of the three appear to be strongly aligned with the Quincy Clan, which is strange considering the fact that the Soul King is supposed to remain impartial.
Mimihagi: The Right Arm
As the personification of stillness, Mimihagi is a living shadow that resides within Seireitei and has the ability to terminate development, change or progress. He can also see into the future and is therefore capable of ending an undesirable chain of events before they even begin. These powers are not inherently evil, as Mimihagi can cease the advancement of the disease and the spread of illness, as was done with the young, sick Jūshirō Ukitake of the Gotei 13. Mimihagi, however, takes the inflicted body part from its host, subsequently forming an unbreakable connection with that person. The "healed" body may now also become host to this shadow demon, although Mimihagi must be called willingly into the vessel - which is inevitably a voluntary suicide mission.
Pernida Parnkgjas: The Left Arm
Pernida Parnkgjas is an official member of the Quincy Clan, falling under the 'C - The Compulsory' ranking in the Anime's Wandenreich's Sternritter. He is Mimihagi's counterpart, as Pernida is responsible for encouraging growth and is capable of stimulating accelerated evolution. The Left Arm, however, uses his powers for evil, by absorbing other beings in order to inherit their skills and traits, advancing his own development. By using his attack 'The Compulsory', Pernida can extend nerve endings from his fingertips that are powerful enough to rip almost any opponent to shreds. If not set on instant death, these nerve extentions can control their victim like a puppet, contorting their individual limbs past the point of destruction.
Gerard Valkyrie: The Heart
Another Quincy member, Gerard is classed as 'M - The Miracle' and has a unique ability to manipulate the probability of success. Staying true to his nickname, his powers are strengthened when the task seems impossible and only a miracle could 'save the day'. Additionally, Gerard can physically manifest people's thoughts and feelings, and use those sentiments to empower himself or others. During battle, the Quincy will convert his opponent's fear into a personal power boost, physically bulking up in the process (aka Godly Size). Gerard can also siphon his accomplice's hopes into his Spirit Weapon, Hoffnung, and amplify its power potential significantly. Oh, and if that isn't all OP enough, he can also sprout wings and fly- hence the name Valkyrie.
The Soul King And The Shinigami
It appears that, for most of his existence, the Soul King has remained uninvolved in the affairs of man. For whatever reason, the Soul King's body was mutated and the detached body parts became individual cognitive beings. Pernida and Gerard chose to join the Quincy Clan, adding further potency to their already formidable army. Possibly to inhibit any further imbalance in the scales of power between the Soul Reapers and the Quincy, the Shinigami decided to "safeguard" the Soul King, essentially imprisoning him within a fortress of bakudo spells. A Royal Guard is set up to ensure his 'safety', with each Shinigami's bones being transformed into a key that grants travel into (or out of) the Soul Palace, However, these severe protective measures have inadvertently inspired hatred from the Soul King, who now called Shinigami his enemy.
The Soul King And The Quincy
Not only is the Soul King the supposed ruler of Sereitei, but he is also the father of menacing Quincy King Yhwach - however, there is no love lost between these two. Yhwach makes it his purpose to destroy his father, and subsequently eliminate the barriers that separate the human realm from Sereitei and Hueco Mundo, therefore eradicating the concept of death completely. Surprisingly, the Soul King's own flesh, Gerard and Pernina, join his blood, Yhwach in a fight for his destruction in the Bleach: Thousand-Year Blood War Arc. If the Soul King truly is virtuous, (or even if he is impartial) how did he create such evil in the world, and why does he inspire no loyalty from his 'offspring'? Maybe the Shinigami had the right idea after all, by gaining control of the situation before more auspicious appendages were let loose on the world.