Comments like this, and him lamenting over never receiving a heart, imply to me that Saix was unable to grow his own heart. Which is probably why he was taken over by Xehanort…at least that is what I believe now. All the members who were not used by Xehanort because they developed “too much of a sense of self” clearly had grown hearts of their own.
Idk, just rambling :P
(Source: akushion)