Kotona Takashina is a high school student in Tokyo, part of Ichi no Kuni. She shares a close friendship with Yusuke Ninomiya and a romantic relationship with Haruto Ichihara. Kotona experiences moments of jealousy toward Haru's admirers, reflecting adolescent concerns, and demonstrates consideration for others—such as regretting a path choice that excluded her wheelchair-using friend Yu during an outing.

A pivotal incident occurs when Kotona is stalked and stabbed by a Black Banner Assassin in an alley. This attack binds her fate to Princess Astrid of Ni no Kuni, a parallel world, revealing them as soulmates who share traits like a preference for sour oranges. The assault triggers events where Yu and Haru traverse between worlds, unveiling Kotona’s critical condition: a malignant tumor with a three-month prognosis. Her health deteriorates rapidly, necessitating sedation, while her well-being mysteriously links to Astrid’s cursed state in Ni no Kuni.

Kotona’s survival depends on resolving the conflict in Ni no Kuni. Following antagonist Galeroth’s defeat, her tumor is surgically removed. This victory exacts a cost: Kotona loses all memories of Yu’s existence. Despite this memory alteration, she resumes her life with Haru, continuing their relationship and daily routines with apparent happiness, free from the interworld threats that once endangered her.