Koko is the human partner of the mamodo Zofis in the battle to decide the next Mamodo King. She grew up in poverty in a small village, where her family was forced to steal to survive, earning her an unfair reputation as a thief among the villagers. Despite this harsh treatment, she remained a resilient and kind-hearted person dedicated to her studies and work. She once saved Sherry Belmont from drowning during a suicide attempt, an act that forged a deep friendship between the two. Koko gave Sherry a reason to live, and they promised to find happiness together despite their vastly different upbringings. When Koko was accepted into a prestigious university, the mamodo Zofis appeared and offered her his spellbook to become his partner. She refused, unwilling to hurt anyone. In response, Zofis used his powers to forcibly take control of her mind, amplifying her painful memories and corrupting her heart to serve his own ambitions.

Before Zofis's influence, Koko was genuinely happy, gentle, and encouraging. She chose not to succumb to the despair and cruelty inflicted upon her by her neighbors. After her mind was manipulated, her personality twisted into a cold, cruel, and sadistic version of herself. This corrupted Koko derived a warped happiness from power and destruction, taking pleasure in hurting those weaker than herself while claiming that this was her true self finally allowed to emerge. Her original motivation was to study hard and achieve a better life. Under Zofis's control, her primary drive became revenge against the villagers who mistreated her and against Sherry, whom she unfairly blamed for her lifelong feelings of inadequacy. She embraced Zofis's offer of power as a chance to destroy her past and create a new life of luxury.

Koko serves as a major antagonist alongside Zofis during the Stone Tablet arc. Her forced corruption acts as a central emotional pillar for Sherry's character, driving Sherry to fight hard in the tournament to save her friend. Koko's tragic fall from grace represents a key conflict involving the manipulation of humans by mamodo, contrasting the genuine partnership Zatch and Kiyo embody. The most critical relationship in her story is with Sherry Belmont. Their friendship, forged through a life-saving act of kindness, is the heart of the conflict in their shared storyline. Sherry's love for her friend is severely tested by Koko's false confessions of lifelong hatred. The real Koko, trapped inside the corrupted personality, subconsciously leaves clues for Sherry and eventually cries out for her friend's help from within the darkness. Her relationship with Zofis is one of abuser and victim, as he steals her will to serve his ambition.

Koko's character development follows a tragic arc. She transforms from a downcast but kind girl into a villain. During her battle with Sherry, the real Koko briefly surfaces, begging to be saved. After Sherry and Brago defeat Zofis, Koko falls into a temporary coma. Zofis explains that he created a diabolical alternate personality within her, meaning that when she returns to her original kind self, she must live with the memories of the terrible acts committed by her corrupted persona. As Zofis's partner, her notable ability is the capacity to read and activate his spells from the spellbook, channeling his dark flames into devastating attacks. Her inner strength is also remarkable, as her intrinsic kindness continued fighting against the forced mental manipulation from within.