Kaya Saimori is the younger half-sister of Miyo Saimori and the biological daughter of Shinichi Saimori and his second wife, Kanoko. Born roughly three years after Miyo, Kaya grows up as the cherished and favored child of the household, in stark contrast to her older half-sister, who is treated as a servant. Kaya has a beautiful appearance, characterized by long, wavy blonde hair and pink eyes, and she favors ornate clothing that reflects her status.

Kaya’s personality is defined by her upbringing as a spoiled and arrogant young woman. Having been praised and indulged by her parents from a young age, she develops a deep-seated belief in her own superiority, particularly over her half-sister Miyo. She is emotionally volatile, two-faced, and takes a cruel, sadistic pleasure in oppressing and belittling Miyo. Her attitude is not merely passive disdain; she actively torments her sister, following the example set by her mother, Kanoko, who harbors a hereditary resentment toward Miyo as the child of her husband's first wife. Kaya's fiancé, Kouji Tatsuishi, also becomes a target of her disrespect and nasty behavior, as she shows him little care or affection.

A powerful driving force behind Kaya’s actions is her intense envy of Miyo. This jealousy is not born from a lack of privilege but from a compulsive need to possess or destroy anything good that comes to her half-sister. When she learns that Kiyoka Kudou, a powerful and handsome commander, treats Miyo kindly, Kaya becomes infatuated with him and is determined to steal him away. Her desire to switch fiancés is less about genuine affection for Kiyoka and more about her inability to tolerate Miyo having something superior. This envy culminates in her active participation in a plot to have Miyo kidnapped, during which she and her mother torment Miyo in an attempt to force her to break off her engagement.

In the narrative, Kaya serves as a primary antagonist in the early story, embodying the cruel and abusive environment from which Miyo escapes. Her relentless bullying and schemes create the immediate conflict that threatens Miyo's newfound happiness. Key relationships define her role: her alliance with her mother, Kanoko, in abusing Miyo; her contemptuous dynamic with her father, Shinichi, who enables her behavior; her engagement to Kouji, which is a tool for hurting Miyo rather than a genuine partnership; and her obsessive rivalry with Miyo, whom she views as an obstacle to her own happiness.

Kaya undergoes a significant, albeit forced, character development following her failed scheme. After Kiyoka rescues Miyo, he publicly rejects Kaya, delivering a devastating blow to her pride by declaring he would never marry an arrogant woman like her. As punishment for her role in the kidnapping, the Saimori family falls from grace. While her parents are sent to the countryside, Kaya is specifically sent to a strict, ordinary household to work as a live-in servant, a karmic punishment that forces her to experience the same harsh, labor-intensive lifestyle she once forced upon Miyo. This fate marks a complete reversal of their positions, leaving her humiliated and stripped of the status and privilege she once wielded as weapons against her sister.

Kaya possesses a supernatural ability known as Spirit-Sight, which is common among the Gifted families. This power allows her to see Grotesqueries, malevolent spirits invisible to normal humans. Additionally, she has learned to manipulate shikigami, or paper familiars, a skill that anyone with Spirit-Sight can develop. Using a technique called Share-Sight, she can remotely view locations by connecting her vision with these familiars, an ability she uses to spy on Miyo after she moves to the Kudo estate. While she is praised for having this gift, which Miyo was believed to lack, Kaya’s abilities are considered unremarkable, and as a woman of her social standing, she was never trained to use them for combat or advanced supernatural feats.