The Spellcaster part is mostly flavor - the character in the image is a member of the Kirin Tor of Dalaran, a city of magi in World of Warcraft. She's a high ranking member of their military and is decked out in spellbreaker's armor.
As for your suggestions, they're greatly appreciated. I designed it with something like Decode Talker in mind; you synchro summon her in tandem with him to mitigate the cost of freely using his effect to negate things and further protect her. I'm not entirely sure how I could design her effect to accomplish what I want while still working with existing wordings and rulesets.