Celestial

Evolved from a Light spirit who has the trait ShadowAffinity or a ShadowSpirit who has the trait LightAffinity, Celestial don't evolve further

The requirement for evolution may be a bit weird since you can't breed with spirits, so is recommend using Hildy and Lyra to summon spirits of Light and Shadow until one comes out with Catalyst which will allow them to fusion and grant the required trait.