How to Get Keroppi's Lost Luggage | Hello Kitty Island Adventure

You can upgrade your stamina by crafting a Gold Apple. To do this, you need Apple Slices. The slices can be found around the map in treasure chests or earned as rewards by leveling up friendships. Personally, it took me about two days to do it. Some Apple Slices become available after upgrading your stamina by one level (I mean that is more possible to collect them after that, making it more easy.)