Just to clarify, what Dejnov explained is Lugh's skillset, not LAthena's.
LAthena is a tank that can cover for 3 turns, improve her own defense + dark resist and has a 25% dmg cut ever 3 turns for the party. For now she is a solid protector for the likes of Vishnu who need time to wind up so they can go nuts. Later down the line she gets some improvements which make her arguably one of the best tanks in the game.
If you use a weapon in a grid that is the favored weapon of a Hime in your team, she gains additional 20% of the weapons stats as bonus. It is a min-maxing-option if you want to absolutly optimize.