Last 1/3 of the game, Yggdra is overpowered IMO. Yggdra Union spoilers...
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She can use any card that has an ace restriction only, and her two class cards are awesome - one does critical damage and the other is an auto-win card.
Did someone already say Yggdra?
I think Maria from KitN is ridiculously powerful, too. She has a huge attack range, can use any weapon, can move, jump, and attack in any direction, and has a lot of attack power. Need I say more?