Mother and Child Isles
They say that inside the ring of rock
The Mother half of the island can only be reached after rescuing Aryll from the Forsaken Fortress and learning the Ballad of Gales. Inside, Link can find a fairy haven and the Fairy Queen who proceeds to gift him the ability to use Fire and Ice arrows.
The Child portion of the island lies a short distance from the Mother portion. It can be reached via the boat. Upon this island Link is able to find the Korok named Rown and the one of the Traveling Merchants who plays a role in Zunari's sidequest.
A Blue ChuChu can be found on the Child Island.