Kodai Arima

Takuya's father, Kodai Arima, is a scholar with a PhD in history. He put forward his own unique hypothesis that, through events such as the relocation of the capital to Heian-kyo, the establishment of the Kamakura shogunate, and the founding of the Edo shogunate, Japan experiences major reforms approximately every 400 years. He also believes that around 400 AD, a people called the "Takano Amahara" invaded and unified Japan. Kodai, having challenged the theory of his academic mentor who was an authority in the scholarly community, lost his place in academic circles. However, his 400-year cycle hypothesis drew the attention of Kouzou Ryuuzouji, which allowed him to continue his research at Sakaimachi Academy.

Kodai is convinced that both the 400-year cycle hypothesis and the secret of the "Takano Amahara" are hidden in Sakaimachi, and he focuses on a strange rock formation called Ken-no-Misaki. During his investigation, he meets Yurika Imagawa, a physicist who also takes an interest in Ken-no-Misaki, and proceeds to investigate together with her. Through Yurika, he meets Keitia, eventually marrying her and having a child, Takuya. However, after Takuya's birth, Keitia sadly takes her own life, but not before revealing her origins and homeland that support the 400-year cycle hypothesis. Kodai later marries his former student, Ayumi Arima. Still, in pursuit of historical truth and to reunite with his beloved wife, he fakes his own death in an accident and journeys into the "Boundary of Dimensions."