Mirai, Kun Ota’s younger sister, upends her family’s equilibrium with her birth, igniting her brother’s turbulent emotional journey. As an infant showered with parental attention, her presence fuels Kun’s jealousy and sense of neglect, propelling him to cross paths with a teenage Mirai from the future. This older version becomes his guide through time-traveling odysseys that unravel their family’s layered history. Together, they witness pivotal moments from the lives of their parents, grandparents, and great-grandparents, uncovering how intergenerational choices forge the contours of their present.

The teenage Mirai steers Kun toward growth, bridging his resentment to empathy. Journeys into the past expose him to their mother’s childhood loneliness, their great-grandfather’s life-altering wartime injury, and their father’s youthful insecurities—each revelation nurturing Kun’s understanding of responsibility. Future Mirai’s sporadic interventions, like aiding with a family heirloom’s preservation or facing existential dread at a train platform, weave recurring motifs of kinship and reliance. Her exchanges with Kun underscore the inevitability of change, urging him to embrace his evolving role as an older sibling.

Though infant Mirai remains passive in the present, her symbolic duality—tying Kun to the family’s roots and future—anchors the story’s meditation on legacy. The family garden, a threshold between eras, becomes the stage for these revelations, illustrating how individual actions cascade across generations. Kun’s eventual acceptance of Mirai mirrors his maturation, transforming rivalry into a recognition of unconditional familial bonds. The conclusion cements her dual significance: a cherished sibling and an emblem of kinship’s enduring thread.