Koyasha wrote...
Well, Wynne seems to have retained herself, basically. Of course, we don't really get to know her before and after, and she wouldn't exactly be the most impartial judge of whether she's changed or not. Still, if she's any indication, such a melding wouldn't necessarily mean the destruction of the self of either of the two individuals involved. She certainly seems to behave as though the spirit is a separate entity joined with her, not as though she and the spirit have literally become one mind and person.
As you say, as we have no idea what she was like before, we don't know if she's changed at all, we don't know how long she's had the spirit in her (though it seems to have not been that long) and we don't know what type of spirit it is.
If a caring and compassionate person has a spirit of compassion in them, there wouldn't be a major personality shift. If a caring and compassionate person has a rage demon in them, even if it's not in control of them, you might see a strong change in personality.
Moreover, I didn't say that they'd merge until they were unable to think of themselves as anything but a single being.
Have you ever seen Star Trek? In it is a species called the Trill who are able to become host to another organism with a vast memory and its own personality, which is also called a Trill. I imagine a spirit inhabiting a person that it hasn't taken control of like that.