Hikaru Saitō, Fuuka Saitō’s older brother, anchors their story’s central dynamic. Two years before the main narrative, he brushed off her childhood marriage pact as a fleeting crush, humoring her with a lighthearted promise. This casual agreement now haunts him as Fuuka’s puberty-driven growth spurt leaves her towering over him with a strikingly mature physique. Her unwavering romantic gestures, unchanged since childhood, clash with her adult-like appearance, sparking awkward misunderstandings at home and school.

Hikaru grapples with the dissonance between Fuuka’s youthful earnestness and her mature presence, his earlier dismissiveness crumbling under her relentless advances. Her actions—teetering between sisterly affection and romantic pursuit—leave him increasingly flustered, exposing the fallout of his past nonchalance. Their shifting dynamic underscores the clash between nostalgia and present-day discomfort.

Now a university student, Hikaru navigates a life stage far removed from Fuuka’s middle school world. The age gap magnifies their mismatched emotional maturity and societal expectations, layering their interactions with equal parts humor and tension.