Mirrim, of the Double Blades

Whenever you add a +1/+1 counter to a creature, double it.
Whenever a player other than you adds a +1/+1 counter to a creature, they may double it. If they
do, add a +1/+1 counter to a creature.
Whenever a player other than you creates a
creature token, you may add that many 2/2
Spirit Cleric creature tokens.
Whenever a creature token you control dies, you
may sacrifice a creature token. If you do, you may return a creature or plainswalker card
from your graveyard to your hand.