Which characs...?

Okay, I'll do the comparisons. Zidane: no contest, he HAS to be in your party. Vivi versus Freya: Vivi's magic does substantially more damage than Freya's attacks or special abilities. He does get knocked out VERY easily, but being able to do 9999 damage EVERY time you bounce Flare off of a character with the Reflect 2x ability is too good to pass up. Dagger versus Eiko: Well, you need one of the two, simply because a white mage is a necessity. Eiko can shrug off substantially LESS damage than Dagger, Dagger can use Ark (which will do tremendous damage to Necron), and you get Dagger early in the game, so boosting her levels and abilities is a lot easier. Amarant versus Steiner: this is just a personal preference of mine. Amarant's Aura skill comes in handy because is casts Regen and Auto-Life on any character you target it on. Amarant also takes a heap of damage before going down, and his attack power is equal to that of Steiner's. The only advantage Steiner has is that if you use his Shock skill with the Ragnarok equipped it does 9999 damage every time. I prefer Amarant's Aura ability mainly because it provides good insurance in later battles.