The answer is Mindy and her Fayth.
Essence and form conflicted: Mindy and her Fayth have very different personalities.
Brash yet painfully shy: Mindy fights aggresively, but her Fayth is too shy to even look up at Yuna and Tidus.
Demure yet disobendient: On occasion Mindy (and her sisters) will simply ignore Yuna's commands in battle.
The smallest yet powerful: She's the smallest of the Magus Sisters, but her Passado attack is one of the strongest in the game.
N8V_Tidus25, you thought of the answer first, but blitz_king10 identified the name correctly. So the one that posts first gets the next riddle.
The image below is of the Fayth of (L-R) Cindy, Sandy, and Mindy.