Nix, Winter Witch

Hexproof, Changeling
Whenever you cast a noncreature spell, untap
Nix, Snow Shifter.
{s}{s}, {t}: Nix, Winter Witch becomes a copy
of target creature until end of turn, except its
name is still Nix, Winter Witch, it's legendary
and snow in addition to its other types, and it
has hexproof and "Whenever you cast a
noncreature spell, untap this creature."