Even if he hadn't been stripped down to human, he wouldn't have sensed Aizen coming, not unless Aizen wanted him to. Still, he's thought Aizen dead for well over a year, so when he hears the voice it turns his blood to ice and he freezes in place.

He shudders, like a prey animal feeling the breath of a predator on its neck, and turns his head slowly to let his eyes confirm what his ears and instincts have already told him. The fear is different in this body, with a heart to fuel it. It wasn't the fear of defeat, it was... deeper. Wider. Obnoxiously expansive.

He clenches his jaw, pulls on a facade of arrogance. "I heard Kurosaki kicked your ass," he replies, slowly getting to his feet and glowering at Aizen and his... weird little hollow. "These more of your experiments?"