Narifumi Hosokawa is a Japanese high school student preparing for his school's culture festival with the Kadomori twins, Shizuku and Kasumi. He stands 170 cm tall, weighs 64 kg, and possesses brown hair, brown eyes, and blood type B.

He is in a relationship with Kasumi Kadomori, though their interactions remain tame. Despite this commitment, he proves susceptible to Shizuku Kadomori's seductive advances and fails to resist her manipulations. This lapse draws him unwillingly into a conflict surrounding the Black Gate—a mystical portal to an alternate world holding immense power. Shizuku’s betrayal in opening the gate violates her family’s ancestral oath to protect it, forcing its defenders—including Kasumi, other female students, and a teacher—to pursue her into the other dimension. Narifumi accompanies this group.

In school settings, he displays laziness and a lack of academic diligence, frequently clashing with his class representative, Mizuki Komiyama, over his unmotivated behavior. However, when confronted by interdimensional threats stemming from Shizuku’s alliance with new adversaries, he demonstrates unexpected adaptability and reliability in protecting others. His involvement becomes pivotal as the group navigates the dangers of the alternate world to prevent disruptions in both dimensions.