LogFAQs > #976424898

I'll say this(which can be said WITHOUT spoiling things):

-Every Eidolon acts as a "reality check" for the given character. WHY it awakens at that particular moment is relevant to the character's personal development. The game will explain the "checks" so I won't go deeper, though if you pay attention to the character personalities, you'll likely pick up the reasons before the game tells you.
-The party makeup is also "plot-based intentional." Everyone is paired with each other for particular reasons. Again, plot will explain this thoroughly. Because the characters are meant to be more grounded, they're intended to gain things from each other.

Can't comment on anything else without spoilers so it is left there.