Saeko inhabits a village destined for submersion under a new dam. She forms a profound bond with Yuta after his unforeseen arrival from the future, deepening during his stay in her timeline. Orphaned young and raised by her grandmother, she maintains inseparable roots within her community. Her childhood centers on adventures with Kenjo, another village youth, and Yuta—preparing for the settlement's final summer festival, swimming in local waters, and watching fireflies after dusk. A concealed heart condition critically shapes her worldview and daily existence.

Her evolution involves uncovering her family’s ties to both Yuta and the enigmatic figure who displaced him through time, revelations that redirect Yuta’s purpose beyond awaiting his return. Symbolically intertwined with fireflies—embodying transient beauty and endurance—she mirrors her own fragility and the village’s looming erasure. Her narrative culminates in surviving her health ordeal and reuniting with Yuta in his future era, enabled by the temporal disruption events.