Edda.
After Tam-Tara HM, the whole prospect of her crazed spirit haunting places (as seen in the final cutscene and in the city-states) left a lot of potential for future creepy fun. Instead, she was shoved in here as an unwilling puppet of an antagonist from another game series, 'cured', and the story was closed. While I loved her new theme-song and her new look was good (though a bit of a downgrade), nothing about her here was really creepy, and that potential was squandered. Hell, had they handled it right and just dropped the Nybeth concept, leaving her as the main antagonist, it still could have worked pretty well, but they just had to get that Tactics Ogre tie-in since they went with PotD. :-\
I still say if they wanted to bring her back as more than just a spirit, she would have made a great Necromancy trainer for conjurers. :-(