Immediately after obtaining Serene's Pendant, go further into the tree. You will meet an injured Harpuia. You can choose to either kill or spare it. Spare it, and you will get her undying gratitude as well as "Happy's Plume". This item when used by Ein will do close to 200 damage to the enemy with the highest HP. Happy's Plume can be used ten times before it shatters.