Minerva McGonagall

Professor (Whenever a nontoken creature enters the battlefield with a lower power or toughness than this creature, put a +1/+1 counter on it)
{w}: Exile Minerva McGonagall and return it to
the battlefield transformed.
{r}{u}: Target artifact you control becomes an
artifact creature with power and toughness
each equal to its converted mana cost
until end of turn.