Alright I'm gonna say it: Yhwach From Bleach the villain from Bleach's Thousand Year Blood War Arc.
His Imperial Majesty, Ywhach, Father of the Quincies, Emperor of the Wandenreich.
"The Confined King, 900 years to regains his pulse after 900 years, his mind after 90 years, his strength after 9 years, and the world in 9 days."
-- The Quincy Song Prophesizing Yhwach's return.
Originally you're given the perspective and history of the world of Bleach from the perspective of Soul Society and the Soul Reapers who live and operate in it. The Soul Reapers/Shinigami are presented as noble guardians of peace and protectors of the balance of the world. Though the cracks in this narrative begin to show once you actually see Soul Society up close. Its government under the Central 46 is quite corrupt and incompetent while its own internal structure is heavily stratified and stagnant.
Then there's the issue of the Quincies who were a race of humans with spiritual energy that allowed them to fight hollows. Though unlike Soul Reapers the Hollows they defeat aren't purified and returned into the Soul Cycle, but destroyed. The initial explanation was that this led to conflict between the Soul Reapers and Shinigami which led to most of the Quincy being wiped out leaving a small remainder whose numbers dwindled over the centuries.
If you guys are planning on watching Bleach when it returns later this year or next year you guys will probably really enjoy it as some of the high points in this arc really give the acclaimed Soul Society Arc a run for its money despite the rushed ending. I have no doubt the anime will probably fix a lot of those issues as most of those issues were caused by Kubo's health failing forcing him to end the manga early.
Yhwach is essentially the father/progenitors of the Quincies who a thousand years earlier led the Quincies in a bloody conflict against the Shinigami hence the name of the arc: The Thousand-Year Blood War Arc. You learn here about the true nature of the Soul King and Soul Society.
The first seeds of doubt about the nature of Soul Society and the Soul King are planted into the readers'/viewers' minds in the interactions between Urahara and Aizen. A notable example of this is when Aizen is finally defeated and sealed by Urahara.
Aizen is uncharacteristically engaging in an unhinged rant which is very unusual for his character as he's known for his calm and rational demeanor. It makes viewers/readers wonder what exactly was so bad about the Soul King for Aizen to hate him this much (t
he anime shows this even more as Aizen in a fit of rage starts violently ripping off parts of the seal as its envoloping around him). Funny enough while Urahara accepts the Soul King's rule he similarly doesn't necessarily refer to him with much respect either. From what we're told by other characters, the Soul King is an absolute ruler whose spirutual presence is felt everywhere, though he doesn't actually directly rule soul society with his power actually being delegated to the Central 46 which functions as essentially an assembly of nobles with the Gotei 13 serving to enforce its rules. The Soul King however resides in a separate "dimension" of sorts guarded by the Zero Squad
During the Thousand Year Blood War its actually revealed that the Soul King is actually not actually a real King. He's basically a tool used by the Shinigami to propagate their corrupt system of rule for millennia. With the actual Soul King essentially a corpse kept in a state between life and death inside some sort of crystal suppressing his power. Originally the state of the world was such that all the various realms (Hueco Mundo, Soul Society, the Living World, etc) were all one before being split apart through the Soul King's power (he's essentially a divine being or sorts). In this primordial world souls and humans existed alike before the sudden emergence of hollows which began devouring humans. In response to this chaos the Soul King emerged and with his power (its seems to be quite similar to the Quincy powers) he used to defend the weaker souls and humans from being consumed by these monsters.
The other 5 Great Noble Families then concerned about the Balance of the world sealed up the Soul King with him being the lynchpin that maintains the artificial separation of the world. Though the Shiba clan ancestor protested against doing so and offered himself up as the lynchpin. Though he eventually relented as he wanted some sort of purification for the Hollows instead of having them wiped from existence. Fearing that the Soul King would escape, they add to their betrayal/Original Sin by mutilating the Soul King, rendering him essentially a quadriplegic. Inside the crystal he's essentially trapped in a comatose state for millenia as nothing but a tool for the Shinigami. The Shinigami essentially introduced death as a concept in the world creating the reincarnation cycle for souls.
The original form of the Soul King.
This was where Ywach the Quincy Emperor comes in. Ywach is the Soul King's son and during the original Thousand Year Blood War, he essentially led a revolution/uprising against the Shinigami's rule. In the world of the living, Ywach conquered large swathes of it establishing his "Empire of Light." From there he invaded Soul Society set on restoring the old world where death didn't exist and to end his father's humiliation.
Yhwach however was defeated by Yamamoto (the head of the Gotei 13) with his monster Bankai. The result of the battle essentially left Ywach comatose and powerless for almost a thousand years. Furthermore the Shinigami (the Old Gotei 13 captains were psychotic and bloodthirsty murders only brought in line through Yamamoto's overwhelming strength) then went on a campaign of extermination against the rest of the Quincy all but wiping out Ywach's bloodline while his Empire fell to ruin. The survivors of the Lightreich (Empire of Light) essentially flee with their fallen Emperor to a separate realm establishing the Wandenreich (Walled Empire) awaiting Yhwach's return.
Yhwach during the Thousand Year Blood War Arc ruthlessly takes out the Shinigami in the Sereitei and the Zero Squad guarding his father's corpse essentially. Ywhach then mercy kills his father thereby "ending his humiliation," and absorbs his father's power becoming the new Soul King. Yhwach tries to merge the separated world back together in its original state, but ultimately fails as he's killed by Ichigo. Though the worst part is that Yhwach as the new Soul King had his corpse enclosed as the new lynchpin upholding Soul Society. This is a fate worse than death. The worse part is that if things had been a bit worse with Yhwach's corpse not really salvageable, the Zero Squad and Shinigami were planning on secretly making Ichigo the new lynchpin.
Yhwach honestly wasn't that bad initially until probably after his original fight with Yamamoto which sees him become twisted and hell-bent on revenge during the TYBW Arc. Despite his selfishness, a world where Yhawch probably wins is likely a much better one than the current one. The existing system under the Shinigami was failing with there being so many hollows outnumbering the Shinigami. Their devotion towards "the balance of the worlds" comes at the cost of the many innocent human souls devoured by the hollows. Then there's the fact that Soul Society itself is quite stratified with most of its souls living in abject poverty in worse conditions than the people on earth.
The world Ywhach envisioned was essentially free of death as well. With the quincy the hollows could probably be kept more easily at bay as well. For all the criticisms that people like Ichibe made of the original world when he saw it as stagnant, the same could be said of their system as well while the Quincy have advanced massively in terms of technology and organization resembling a modern military force as opposed to the Gotei 13 which resembles Feudal Japan. Yhwach had flaws and while he may have been villainous, he still had a point at the end of the day.