Ryuuguu Fantastica

During the last Houryou festival when it is said that the oceanic Gates of the Ryuuguu (Dragon Castle) open every 7 years, Yura found Nao on the beach. Without any relations, Yura takes him in as her younger brother. Thus they happily grow up together for the next 7 years, but now that the next Houryou festival is looming, Nao seems to be acting strangely...