First, get the Cabana Deed from the school teacher Then collect 40 Joy Pendants (on top of the 20 you gave to the teacher). You can steal Joy Pendants from Bokoblins, so be sure to visit the outlook platforms on the ocean. Visit the schoolhouse again and present her with them. If you have more than 40, she'll take those, too. As a reward, you will receive the Hero's Charm. You can equip this mask from the stats screen that shows your weapon, songs, and Triforce pieces. While it's equipped, you can see your enemies' hitpoints.