Siren of the Fanged Coast

For effects that check which player put counters on the entering creature, the player chosen to pay tribute puts those counters on it, not the creature’s controller.

Players can’t respond to the tribute decision before the creature enters the battlefield. That is, if the opponent doesn’t pay tribute, the triggered ability will trigger before any player has a chance to remove the creature.

If the opponent pays tribute, the creature will enter the battlefield with the specified number of +1/+1 counters on it. It won’t enter the battlefield and then have the +1/+1 counters placed on it.