Is the Soul King a Shinigami? The Soul King is shown in the Thousand Year Blood War arc, though only briefly. Not much is explained about him, so things like what he can do and what his job is are a bit vague. Is he a Shinigami? If not, what is he?
Like Madara mentioned, it's really impossible to say at this point. From what I remember from that brief moment in the manga, the silhouette of the Soul King kind of looked like Aizen's pre-butterfly state.

Aizen also called "it" a "thing," so it might not be a Shinigami.