
Originally Posted by
Sephiroth0812
Who says that Isa is with Xehanort's XIII darknesses willingly?
It's pretty possible that Isa's consciousness and will is already so much overwritten and subdued by the "Xehanort"-fragment inside him that he can't really oppose Xehanort that much anymore.
If anything, Saix's reaction in KH2 actually shows that he, despite being one of the "successful" XIII darknesses, wasn't into the whole plot and didn't knew all about the true purpose of the Org.
I even tend to assume that Xemnas and Xigbar were actually the only ones who knew the whole truth.