Go to the place on the map, slightly South-West from the center, where it says you can't enter the forest.

When you get this message you should see a fortress in the distance. Enter the town slightly to the south, and enter one of the civilian houses.

Talk to the mother there and she will ask you to rescue her kidnapped daughter. Exit the town and summon White Ray.

Fly over to the fortress and exit White Ray from there. Enter the fortress and rescue the girl and return her to her mother. You will find the girl is enamored with Raiko. Return to the house several times until the girl gives a secret item to Raiko.