PSIII: I always presumed that Wren was entirely logical, while Mieu was very emotional, like a human.
PSIV: Demi, between both Wren and herself, seemed to have more emotion and feeling... But I can't say that without thinking about the ending where Wren seemed sad about Rika. =( I believe that Wren does have emotions and feelings to some degree, but he does not convey them as easily as Demi might/would.