Serine, Torn Between Lovers

Mingle (When this creature blocks or becomes blocked, flip a coin. If you win the flip, gain control of target creature blocking or blocked by this creature.)
At the beginning of your end step, if you control 2 or more creatures you don't own, choose one of them and sacrifice the other. When you do, put 3 +1/+1 counters on Serine and it loses Mingle.
"When I can finally choose, I will go fro