Two Reasons

"You sound like the Masked Commander right now," Orius Aventine remarked when he heard Iria. "However, you're still injured." He stood up and observed the enemies before he killed the fire. "They won't be able to aim at us easily anymore," he said to Iria and walked out of the small cave.

The moment the second Prince showed up and used himself as the bait, an arrow was released towards him again. However, the Prince depends on his sharp ears and strike down the arrow while Commander Iria released her arrow which was successfully planted on the chest of the assassin.

When Iria witnessed how good the hearing of the second Prince was, she was taken aback. She was always praised for her extraordinary hearing skills but... Orius Aventine was no worse than her!

He might even be better than his brother!