Arikawa Masaomi hails from the modern world as a high school student and childhood friend of protagonist Nozomi Kasuga. He shares an unbreakable bond with Nozomi and his younger brother Yuzuru Arikawa, the trio inseparable in their daily lives. During a rain-soaked day at their school, Masaomi, Nozomi, and Yuzuru encounter a mysterious boy who catapults them across time and space into a parallel world mirroring Kyoto's Heian-period twilight. Masaomi arrives in this realm three years earlier than Nozomi and Yuzuru, tearing the group apart. Throughout their separation, Nozomi sustains contact with him solely through fleeting dreams.

In the parallel world of Kyō, Masaomi aligns with the warring Taira clan (Heike), opposing the Minamoto clan (Genji). He ascends to a fearsome general rank within the Heike forces, operating under the alias "Kaeri naifu" (還内府). This allegiance pits him directly against the Genji faction, which includes Nozomi, Yuzuru, and their allies. His concealed identity and role as a Heike commander ignite profound conflict and emotional turmoil, especially for Nozomi, who uncovers his true allegiance only upon their eventual reunion.

Masaomi radiates charisma, boldness, and reliability. He harbors mutual romantic feelings for Nozomi, though neither voices them openly. Before their displacement, he once demonstrated his care by gifting her a musical antique watch simply because he knew she cherished it.

As a Heike leader, Masaomi forces Nozomi into an agonizing crossroads: protect her friends and allies within the Genji ranks—including standing by Minamoto no Kurō Yoshitsune—or side with her first love, now commanding the enemy forces. This dilemma fuels the central narrative tension surrounding his character in the OVA *Owari Naki Unmei*.