Kyouko Kouda is the daughter of shogi professional Masachika Kouda and the adoptive older sister of Rei Kiriyama, four years his senior. She possesses striking blonde hair reaching her shoulders and clear blue eyes, typically wearing light-toned clothing. Her childhood was dominated by shogi and familial neglect; she relentlessly pursued excellence in the game to win her father's approval, only to face his dismissal based on her gender and perceived lack of talent. This rejection fostered deep resentment, which intensified when Rei joined the household. Witnessing her father focus his attention on Rei's natural shogi talent, Kyouko viewed Rei as an intruder stealing her family and subjected him to sustained physical and verbal abuse throughout their upbringing.

Kyouko exhibits a volatile personality marked by pride and a pervasive sense of emptiness. She often lashes out with intense anger, directing verbal cruelty and manipulation at Rei, particularly attempting to undermine his confidence before important matches. This behavior stems from unresolved jealousy and a desperate, unfulfilled craving for love and validation. Rei describes her as a "glass with cracks," symbolizing her inability to retain emotional fulfillment despite external advantages like beauty or family status. Profound loneliness and fear of abandonment drive her actions, causing her to cling destructively to relationships, including her toxic bond with Rei and an affair with the much older, married shogi player Masamune Gotou. She acknowledges Gotou's flaws yet pursues the relationship relentlessly, accepting material tokens like a wristwatch while fully aware it has no future.

Her dynamic with Rei remains fraught even after he leaves the Kouda household. She repeatedly seeks him out, invading his apartment and attempting to provoke guilt or dependence, revealing a twisted mutual care beneath their hostility. A pivotal encounter with the Kawamoto sisters, particularly Akari, sees Kyouko taunt Rei, insinuating he will destroy their family as he did hers. Simultaneously, she reveals her own financial reliance on her father, highlighting her hypocrisy compared to Rei's struggle for independence. This underscores her jealousy of the warmth Rei finds with the Kawamotos, contrasting sharply with her own isolation.

Moments of self-awareness mark Kyouko's development. She later realizes her blame toward Rei was misplaced, acknowledging he never intended to usurp her family but merely sought belonging. This culminates in a breakdown where she fully comprehends the damage she inflicted on him. Expressing remorse, she consciously decides to distance herself from his life, recognizing she forfeited the right to his companionship. Despite her anguish over impending loneliness—especially as her relationship with Gotou nears its end—she genuinely wishes for Rei's happiness.

Her name, Kyouko (香子), incorporates the kanji for the Lance shogi piece (香車, "kyousha"), reflecting her family's shogi-centric identity, a detail noted in live-action adaptations. Her surname Kouda (幸田) combines "happiness/good luck" (幸) and "field" (田). She was described in childhood as "beautiful as a demon" and held temporary employment.