Emiel the Blessed

{3} Exile another target creature you control, then return it to the battlefield under its owner's control.

Whenever another creature you control enters, you may pay {g/w}. If you do, put a +1/+1 counter on it.
If it's a Unicorn, put two +1/+1 counters on it instead. ({g/w} can be paid with either {g} or {w}.)