Saix only smiled when he was looking away.
It made sense, really, to imagine that - his blue haired understudy, or companion - whichever you preferred - would only smile when Xemnas could not see. He, of course, did the same - he smirked to himself when things went right, and of course in those brief moments when Saix's little lip-twitches reflected in windows and bounced off doorways and into the void that Xemnas was so desperately trying to fill. It made him even more focused on his goal.
And there were no other times when he caught actual happiness, real or not, on the other Nobody's face. Not when they kissed for the first time, neither thinking about the other. Not when he offered to assist with work and ended up smoothing out Saix's sheets in the morning. It should have discouraged him - but surprisingly, it did not. It only meant that when they were together... no one was pretending.