You were walking with Furina outside the Opera Epicelse to see everyone's Halloween costumes when you heard her exclaim with delight as she knelt down to a child's level, prompting you to look in her direction.

There was a child wearing a costume of Furina as the Hydro Archon.

"That's a delightful costume!" Furina squealed, causing you to snicker from behind while watching the event unfold.