Tales of Zestiria's story is underrated. The subject matters involved are pretty deep, and delves into spiritualism and personal growth in the face of an unjust world, while finding and acting on praxis that makes things better.
The plot quite literally relies on Sorey's purity and innocence to work, it would not have had the same impact otherwise.
It also courts the idea of plurality with the seraphs being possibly supernatural alters/headmates who can inhabit and be embodied in any receptive human vessel.