Zelda in Hyrule Warriors: Age of Calamity is an unlockable character. This has been confirmed with the release of the new demo. You can take the Princess into battle to fight off the hordes of Moblins and tons of other Ganon enemies. When you’re playing through Chapter 1, you will eventually encounter the heroine, here’s how to unlock her for play.
You can unlock Zelda in Hyrule Warriors: Age of Calamity during the Road to the Ancient Lab mission. Link and friends must stop a Terminator-esque plot to eliminate Zelda via time travel, yes really. When you fight your way to the ancient lab itself, a cutscene should trigger. You will then be treated to a bit of the story.
After the cutscene finishes, you’re going through an ambush. Link and the bros must fight off a massive force of Moblins and the Guardian itself, the thing sent back in time to attack Zelda. If you manage to fight your way through this disaster Zelda remarks about the idea of fighting back during another cutscene.
After this point, you can select Zelda as a playable character. You can use the D-pad to change between forces in battle, or you can pause the game and press Y to choose a new fighter. You can also unlock Impa before this point in the demo.