I'm somewhat annoyed by the fact that Squad 0 actually exists. Mostly because I had this whole head-canon built up about how there was no Squad Zero, or Spirit King, and the whole idea of a supreme monarch of the afterlife was a sham set up by the Central 46 and maybe Yamamoto.

That's how such an oppressive and blatantly unfair social system like the Rukongai could be sustained for centuries without imploding in on itself, and why there's never been a massive rebellion on the part of the actual Soul Reapers. Whenever the 46 want more power or do something unpopular, they could scapegoat this nebulous and all-powerful figure at the top of the food chain, and all the hatred would be directed at the King instead.

Not even most of the Captains would know, because it was such a useful fiction for getting rid of the most troublesome and powerful individuals. They'd be 'promoted to Squad Zero' - after an elaborate ceremony, Yamamoto would lead them off alone for reasons of security, and he would later return without them. I mean, you've already got the Maggot's Nest, where Soul Reapers who 'retired' are indefinitely incarcerated, but it can't hold Captain-level prisoners reliably. So this becomes the next best option.

Aizen? Aizen was smart and manipulative enough to figure this out. 'No one has stood at the top' indeed. He figured that hey, there's already an elaborate and well-accepted power structure in place to support a supreme ruler, so why shouldn't I BE that ruler? In order to do it, though, he needed to be stronger than Yamamoto, but gaining that sort of power is an easy way to get oneself flagged for 'promotion'. So he had to break away from the Soul Society, and find a place where he could build up his strength sufficiently to move in and take over. Hence, Hueco Mundo, where he had his own army and fortress to protect him while he worked on the last few steps.

Also explains why Kaname Tousen followed him - he was in on the secret, and his moral outrage against the whole system was enough to prevent him realizing that once Aizen was on the throne, he wasn't neccessarily going to reduce the power that the position held.

But no, apparently Squad Zero exists and is made up of five individuals who look completely stupid. Gah.