[Spoiler] How to unlock hidden characters in Age of Calamity Vol.1
This could be a spoiler!
Great Fairies First, you need to complete the first mission of Chapter 4. After that, complete all 4 Hyrule Challenges of Great Fairies.
To complete this challenge, you need to pay 4,500 Rupee, but you can get double of Rupee when you clear it so don't mind. Once you complete it, a new 3 map, called Trial of the Great Fairies will be unlocked.
Clear those 3 maps and then the last map, Fairy Fountain: Battlefield will be unlocked. Clear it, then you'll get Great Fairies.