Due to the bad blood between their people, Iduna kept her heritage a secret from Agnarr. Throughout the rest of her life, she never revealed that she was the girl that rescued him in the forest, but finally confessed her secret to him long into adulthood, shortly before their deaths.

Did Elsa's mom know she was the 5th spirit?

Some have already theorized that Elsa's magical powers were passed on from her mother, but if Iduna had magical powers, then it's possible that she was a Fifth Spirit, too. ... When Elsa arrives at Ahtohallan, she learns that her mother is The Voice - a fact that is confirmed in Into the Unknown: Making Frozen 2.

Is Iduna an Ahtohallan?

Ahtohallan is still a legend associated with the memory of Iduna. Elsa hears the Voice calling her on the balcony, during the charades game, in her dreams before Into the Unknown, throughout ITU, while sleeping in the wagon and while making friends with Bruni.