The Priestess

"Riko"

- Maybe Shrine Maiden is a code-name? It's something that comes up in Orth fairy tales

The Shrine Maiden is a mythical figure thought to communicate with incarnations of The Abyss and warn of impending danger. Cravali and his companion Tepasté are the only individuals known to have communicated with this person. She is the presumed leader of Under Nest.

Nishagora's questioning of Team Riko suggests that the Shrine Maiden is either a White Whistle herself or has one at her command.