The Red Goddess

(from ebookjapan):

Somewhere on earth there is a red-colored person with a red fin. Miya and her father, Dr. Takigawa, go to Rizoa Island to confirm its existence.

The group, together with Dr. Novack and other members of the expedition team, set foot on the uncharted land of Rizoa Island, and to their surprise, an ancient empire of Akaijin appeared out of the mist.

It was a mysterious empire ruled by a beautiful goddess who never ages.

Includes "The Red Goddess" and "The Blue Tunnel".