It looks like the battle GUI isn't working the way it should.
As the game is, at least for me, I can't have three characters at once. I have Cait and Crystallina and I can only use the attacks of whoever's in the first two party slots. Admittedly I don't know if this is already fixed, as I've been using 2.1 instead of 2.3, though I do have to ask in that case if I can continue with the same save when I upgrade.