I think the Emperor is a very special case that Larian created. He has some of his memories which can happen with mindflayers but it goes beyond this for him as he seems to think he is the previous host
and not the tadpole. Contrary to what the wiki says about the mindflayers he does seem to have positive feelings for example he said he was happy before he was taken away
also this could be a lie but he didn't have to say it at all so there was no need to lie.
And he seemed to have cared about the Duke - of course if she really was his thrall it was in a very sick way, but why else having a picture of her in his old hideout or even if you comfort him in the dream scene telling about her and that they were close? To make him appear more human?
I do wonder how he feeds.
Does he manage to kill off some of the honor guard regularly? Or does he leave the prism to fetch something?Strange that we never find anything like Astarions boar.