Characters just kind of get used for whatever earlygame, the further in you get the more specialized they'll become. You'll get to ease into their roles. However, it's easy to overlook how they're properly used in some cases (Nitori and Satori are the most common characters to be incorrectly given a "Terrible" stamp, some people make mistakes with Youmu because of stance shenanigans, and Aya is slightly more useful then most give her credit for, albeit still mostly not relevant)
Coreven:Aya is still nice for a few other things (Stabbing mindflayers or instant deathing them before they move, blinktanking) but yeah it's true that Aya is one of the least generally useful characters- you don't normally need her speed or turn manipulation or blinktanking (Or Breath Reflection, later), and her offensive power and MP is weak once you get a good way into the game.
On Hinotori Skydance, the downside is the non-center slots have a -40% P.def penalty, which is super gross. You probably don't need that much of a tank-centric formation at that point in the game, anyway- but it certainly can be nifty.