Sion keeps most of his focus on the apparition, but he does give a quick glance to Cecil out of the corner of his eye when called out to. The idea of effectively doing nothing rankles him a bit, but at least Cecil's warning is enough to snap him out of his initial reaction.

Not that it gives him much time to do anything else, but... Well, if staying on the defensive is the only option, at least that's still doable.

It doesn't stop his skin from positively crawling at the uncanny resemblance, though.