Kajitsu Momose begins as a green card battler wielding mysterious powers. Initially appearing emotionless, she bears the immense power of Mother Core within her body, significantly taxing her physical health; this burden shapes her every action. She uses her abilities to transport protagonist Dan Bashin into the alternate world of Grand Rolo, initiating the main events.

Her background reveals deeper complexity. She is Yuuki Momose's younger sister, sharing a bond that transcends siblinghood. In a past life, they were lovers—the king of the white world and the princess of the green world—killed and reincarnated. Tragically reborn as siblings yet retaining past romantic feelings, they share a complex and forbidden connection. This reincarnation cycle directly fuels Yuuki's protective actions, leading him to initially serve the antagonist Otherworld King in a desperate bid to save Kajitsu from Mother Core's destructive influence.

As the bearer of Mother Core's light, Kajitsu suffers continuous physical deterioration. Her absolute devotion to Yuuki drives her compliance with his morally ambiguous plans. Her story concludes tragically in *Shōnen Gekiha Dan* when she sacrifices herself, overwhelmed by Mother Core's power consuming her body. Yet her legacy persists. Reincarnated in the sequel *Brave* as Flora, a mazoku (demon) girl devoid of past memories, she embodies a contrasting personality—clumsy, cheerful, and emotionally open—while retaining Kajitsu's essence, symbolized by motifs like butterflies.

An alternate resolution unfolds in the manga adaptation. Here, Kajitsu often finds herself in peril, requiring rescue. Dan Bashin ultimately revives her using Mother Core's power, but this act forces him to become the new Otherworld King, drastically altering her fate.