Swift Serpent, Mirage Ninja

Ninjutsu {r}{r}
When Swift Serpent, Mirage Ninja enters the battlefield, you may play a Ninja from your hand without paying its casting cost. If you do, Detain
it until Swift Serpent, Mirage Ninja leaves play.
When Swift Serpent, Mirage Ninja leaves play,
put two +1/+1 counters on the detained Ninja;
it is no longer detained.