General information Edit
During game, you will be switching between main characters in almost every chapter. Though it may seem annoying by itself, that's not the main issue.
IMO, Eushully did horrible job, balancing parties' combat capabilities. It is especially annoying during first playtrough.
At Celica's side we have "universal soldier" Celica himself and powerful mage Ekuria, and as soon as you'll get access to Tower of Haze (and if you have AP01 installed), another powerful mage with charming smile (Lily) will join you.
Celica has access to five different sets of abilities - two weapon-based and two magic-based, and summon magic.
Ekuria (or how Haishera calls her - "Young Lady-chan" [Jō-chan, 嬢ちゃん]) can use magic to improve party member's capabilities, she can use frost-based magic and "pure" magic.
And Lily can use fire and darkness magic, succubus charms, and blessing/curse spells.
In other words, only with these three you can put your enemies through almost any kind of misery you want - beat them, or just fry them, using Celica's Kill-shots or thunder magic; freeze them or just obliterate them with Ekuria's magic; burn them with fire, defile them with darkness, drew them in despair with Lily's spells.
Oh, and one more thing about Lily - since she's an AP-type character - she'll never leave you via some scripted event - her hellish flames will always be at Celica's side.
On the other hand - Louie is far less versatile combatant by himself (comparing to Celica). And he will be accompanied by only Kārian (mainly), whose both Kill-shots and magic are quite sad. And you'll have to deal with that at least until Chapter 3.
But since here's no choice (aside from SpoilerAL-like things...) - no sense complaining, let's proceed with the prologue.