Dissidia Final Fantasy - No Spoilers
Note: Some costumes are slightly altered to be closer to the artwork of Yoshitaka Amano.
From what I hear (i.e. I could be wrong on this - in fact, I think it is wrong but what ever), their 'crystal' forms are unplayable and are boss characters for the opposing character in their story mode: (ex. Cloud in 'crystal' form would be Sephiroth's final boss in story mode, or Kefka's 'crystal' form would be Terra's final boss -- and so on and so on)
More costume character stuff: