Well let's see now.
Demi - She's got some nice abilities and is funny to see tagging along. Bless her stumpyness.
Alys - She can dish out the pain and take plenty of it too. Plus she has some good techniques and abilities.
Rika - Individual and group healing and buffing is covered and she can hit pretty hard too. Throw in the fact she allows Alys to pull off deadly image and you're sorted.
Rune - Technique madness covered.
Hahn - He actually complements a few of the others well by allowing for some nasty combo attacks.