Every character is useful for different things and you should get whatever character you feel like using in that particular run on both versions.
Celine makes the early game a lot easier with laser beams and starlight which can really help less experienced players and should always be recommended to those who haven't beaten the game before.
Leon has a good spell list, and the best regular attack in the game even if he can't combo in second evolution.
Noel has comparable healing to Rena with better offensive spells, Rena just learns the higher-end heal spells a bit sooner. Also if combined with Leon you can destroy the mid-late part of the story by just mixing acid rain with earth grave (this only works in the PS1 version, however).
Ernest is probably the weakest of the fighters, but his range can still be pretty useful since he gets hit less due to it.
All the others not mentioned are good enough to not particularly hinder you no matter what in either version without any special strategies, though worth noting a lot of people consider Opera better than Ashton due to her ranged KMs and her ability to almost entirely replace Rena with a grinded Healing Star.
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