Comestion, Frazil and Fulmination
Looking for bestiary knowledge.
Ciel has 3-star knowledge in every possible quest in the game. Therefore she's really helpful regarding information.
Plus she has fire, cold, and lightning damage. Also, skills to apply debuffs, and that's gonna help your pawn with bestiary as well. With Holy Boon, she's gonna enchant you with holy damage.
Finally, since she has the 1st tier skills, ends up casting a lot faster and using less stamina. That's the goal.