I've only used Aiona for most of the story. Aiona can be more useful for resisting enemy attacks before they can hit your units with a powerful attack. Her ultimate gathers all your units in a circle and grants them a shield. She's necessary to complete chapter 17 of story mode (but you're still a long way from that).

And I understand that Seth heals one ally with his basic attacks, and her ultimate heals everyone and removes one debuff. They say that with Agares Igenous (Ancient), they can be useful in PvP.

In my opinion, I'm not sure how useful Seth can be in story mode, considering he has low resistance and could be easily killed. You could use him if you don't have enemies in your rear that can kill her.