My memory is a bit hazy on this one, but i think for Carys to join, the MC must have positive karma and you can't have more than one or maybe two corrupted companion at the time. Basically, corruption doesn't matter much on Ruksana, Tishtyra or Clawyn on their own, but since Meredith gets corrupted easily, you need the rest of the team to be in the positive.I still don't understand why she DIDN'T join, I had positive karma of 10+, I helped her and saw ALL the scenes with her, but she never showed up near the castle -_-, I was also upset by the choice of the MAG origin, as for me it's TRASH (even though I caught the Headmistress under lock and key, but apparently there won't be any more content with her) ... in the end it began to seem to me that the BEST choice is probably - \Templar man (at least, you can save a girl\demon from the portal - disenchant her and she will be in the castle)... And then everything is like everyone else in terms of SCENES, about the origin of the Knight and the Thief, I heard that they are also weak...
I can be totally off on this one tho.