Rei Kiriyama, a 17-year-old professional shogi player, achieved fifth-dan status in middle school—a rare feat cementing his prodigy reputation. Tragedy struck early when a childhood car accident claimed his parents and younger sister, leaving him orphaned. Adopted by his father’s shogi rival, Masachika Kōda, Rei’s presence ignited tensions within the Kōda household. Resentment from adoptive siblings Kyōko and Ayumu over their father’s prioritization of Rei’s shogi training fueled a hostile environment, prompting Rei to seek independence at 17, funding his life through shogi winnings while catching up on delayed high school education.

Isolated from peers since childhood, Rei faced bullying and found companionship primarily among adult shogi practitioners. His traumatic past shaped an emotionally withdrawn demeanor, marked by self-reproach and fragile self-worth. Prolonged introspection, muted emotional expression, and social discomfort align with depressive tendencies, while his social awkwardness, muted expressions, and single-minded focus on shogi hint at neurodivergent traits, though undiagnosed.

Rei’s mastery of shogi masks a fraught relationship with the game—initially a lifeline rather than a passion, intensified by Kōda’s rigorous training and competitive demands. Yet shogi becomes a bridge to pivotal connections, most notably with Harunobu Nikaidō, a rival turned ally battling chronic illness, whose dedication disrupts Rei’s detached outlook. Gradually, Rei reevaluates shogi’s role in forging bonds beyond mere survival.

A pivotal shift occurs when Rei crosses paths with the Kawamoto family—sisters Akari, Hinata, Momo, and their grandfather—who offer an anchor of warmth. Akari shelters him during crisis, establishing an adoptive kinship. Hinata’s resilience against bullying reawakens Rei’s faith in compassion, sparking unspoken romantic tension he hesitates to name. His protective actions, like tracking her during a school trip, signal deepening emotional investment.

Rei’s metamorphosis unfolds through therapeutic insights, strategic clashes with champions like Souya, and mentorship from homeroom teacher Hayashida, who steers him toward social engagement. Shedding his self-image as a "cuckoo bird"—a metaphor for displacement and survivor’s guilt—he learns to accept support. By later stages, he actively participates in communal activities and voices gratitude for his relationships.

Landmark moments include reconciling with his adoptive mother, who reframes his departure as protective rather than abandoning, and pursuing education despite initial reluctance. His romance with Hinata, built on mutual encouragement, crystallizes his shift from isolation to interdependence, balancing shogi’s demands with emotional fulfillment through vulnerability and shared growth.