Eida

She isn't a great damage dealer. Even with her anti-mage skills, she doesn't help a lot against them. 
What makes her special is her 5th skill. At max level, it removes "invincibility" from all enemies at battle start. This is just the shields, like Elpaiton has. The benefit is that it makes the 70-shield stages of the Rift incredibly easy from infuriatingly difficult. So while niche, she is amazing at what she does. As a bonus, she also removes ATK/DEF/HP buffs from 2 enemies with the highest ATK.