Hearts are the game’s own currency.
Hearts can be earned in your room by poking your Pakka, picking it up by dragging it up and dropping it, and making it chase your finger on the screen. Another way to earn some hearts is by playing the heart mini game at the movie theater (accessible by movie bus ticket). You can also mine in the cave (cave accessible by bus ticket (bought in store)) for hearts. Even another way is by combining stuff with alchemy with heart gems. The last way is when your Pakka evolves, go into the Pakka book. Then select the Pakka your Pakka is. It will give you hearts the first time you select the picture.
Hearts can be purchased with real money via in-app purchases.