Yukari, Master of Boundaries

Hexproof, Unblockable, Vigilance, First Strike
Whenever an opponent casts a spell,
Yukari, Master of Boundaries gains +X/+0 at
your next combat phase, X being the spell's
converted mana cost.
{1}, Exile Yukari, Master of Boundaries: At the
next end step, return her onto the battlefield
under your control,