Once you have completed the tutorial you will need to choose either Link or Zelda as your starting character. The character you didn't pick is then accessed during the game when you reach Kakariko Village. You will find this asleep in the inn and you will need to wake them up with the Hibiscus Potion which then allows you to switch between Link and Zelda at any Sheikah Stone in the game. This How to Get the Hibiscus Potion in Cadence Hyrule will tell you how to obtain this drink that has magical properties.
To get the actual Hibiscus Potion you need to go to the right of the inn where Link or Zelda is asleep until you reach a graveyard. At this graveyard push the large gravestone away to reveal a set of stairs that lead down to a secret underground dungeon which is Kakariko Crypt. Inside the dungeon you will have to fight enemies as you make your way to the boss room where you will have to face off against a Dark version of either Link or Zelda, the one you face will depend on which character you didn't pick at the start of the game. All you need to do now to get the Hibiscus Potion is defeat them.
These bosses do not have plenty of hearts so you should not have too much difficulty defeating them. If you chose Link you will have the ability to block attacks and if you selected Zelda you will be able to render her invisible for one turn. Both of these abilities help to make this a fast fight to get through. After defeating the respective boss you just need to return to the inn in Kakariko Village to wake up the sleeping character.
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