Hi Steve, I have wondered myself what Aizen meant by calling the Soul King a Thing. I had considered whether the soul King is indeed an inanimate object like a Book of Laws for instance. However, I have come to believe that the Soul King is in fact a living being with mind and conscience, and not a literal thing.
Consider that in Bleach Komamura has a Dog's head, and Yoruichi is literally a Cat. Yet Yoruichi being a cat is still a member of one of SS's founding families. This has led to my current assumptions as to the nature of the soul king.
1) If the SK is not a living being, why does he require a Palace and Five Uber Powerful Captains to Guard him/it. It is quite easy to protect a none living Object, Simply bury it in a place known to few people. No need to move to a private dimension and have 5 Captains who could be protecting Seireitei tied up guarding an empty palace.
2) If A semi-royal Family can be Cats, what's to say that the Soul King is not a Rat, Cockroach, or an ant, even a sentient and intelligent one?
My current thinking is that the SK is a living being, albeit one that is weak and not a life-form held in great regard. I think that this Creature has taken a liking to Kurosaki Ichigo and wants to meet him, those are the orders he has given to the Squad Zero Captains.
THe soul king must have very unique qualities, even if it is weak in battle, and not an impressive life form. I also believe that the SK is peace loving, and disagrees with SS's violent past.
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Thing can mean a lot of... things. For one the spirit king isn't just the ruler of SS he's the "spirit" king. I assume that would be over all spirits. He's a plus, a shinigami, a hollow and everything in between. It'd make sense that he'd be interested in a guy like Ichigo. Wouldn't you describe him in the same way? Except he's got even more attributes than that. He's part supernatural human with the fullbringer techniques and now even Quincy abilities to boot (I wonder if he'll be able to access those powers in his human form like he uses Fullbring, though up until now I haven't actually seen him become a spirit. If you guys remember Rukia stabbed him while he was in his human body... Plot hole I'm sure.). Hes either a kindred spirit to the King, or a legitimate threat. In anyway I believe Ichigo is the real solution to the social issues within Soul Society or better said the Sereitei. He actually cares about everyone, even his enemies. Can buy benevolence.