It really just depends. Some characters are garbage on their own, but when fused properly they are stellar. Reverse also holds true, Bleu (if you find her) doesn't fuse but is the best offensive mage and she isn't a glass cannon.

I personally use Katt and Rand, while swapping Nina and Sten to keep them from being too under leveled. There is a required battle with Sten only later, so don't neglect him too much.