He certainly was working for Aizen - unknowingly, of course. He was the perfect kind of a commander for an insidious villain in the shadows: He always did what the textbook said, never questioned orders (from Central 46), and most likely trusted his first impressions of people until contrary evidence is piled so high he can't anymore see the tip of his own nose. I guess the perfect opposite of Shinji who was too suspicious, yet the same in the one detail that mattered the most to Aizen: Completely predictable.