Harunobu Nikaidō stands as the main rival and close friend of protagonist Rei Kiriyama in the world of shogi. He possesses a chubby build, straight brown hair, black eyes, and often displays pale, flushed skin. His typical wardrobe features polo shirts with slacks or button-down shirts paired with sweater vests, adding coats in cold weather and suits for formal occasions.

Born into wealth, Nikaidō confronted significant health challenges from childhood due to incurable nephrosis syndrome. Despite this, he dedicated himself to shogi, achieving the rank of 4-dan professional player. He first met Rei at a children’s shogi tournament, forging a connection that deepened into a self-proclaimed lifelong rivalry and friendship. Nikaidō persistently declares Rei his "best friend," though Rei remains more reserved about their bond.

His personality blends earnestness, cheerfulness, and stubborn persistence. He maintains a strong public composure during shogi matches, though losses affect him deeply. His dedication to shogi is unwavering, the game central to his identity despite ongoing health struggles. This determination is demonstrated when he plays through visible strain, driven by an intense desire to win.

His health condition leads to periods of physical limitation and hospitalization throughout the series. Yet, it fuels his resolve to advance in shogi, embodying resilience. His rivalry with Rei evolves beyond competition into mutual respect and concern, especially during health setbacks, mirroring real-life shogi player Satoshi Murayama who similarly battled nephrosis syndrome to achieve top rankings before his death at 29.

His character arc highlights perseverance, balancing vulnerability from illness against unwavering passion for shogi and loyalty in his relationships.