Lassitude, Blearing and Maelstrom.
Brigid has 3-star knowledge in every possible quest in the game. Therefore she's invaluable regarding information.
She is focused on applying debuffs (torpor and blindness), which will help your main pawn to build knowledge about enemies as well. But she is also capable of doing great damage with Maelstrom.
With Holy Boon, she's gonna enchant you with holy damage, which is essential for the DLC content.
Finally, since she has the 1st tier skills, she ends up casting a lot faster and using less stamina.