Kaito Shiga is a Japanese teenage student with brown hair and brown eyes. He lost his biological mother in early childhood, retaining only minimal memories of her. His father, Chikara Shiga, subsequently remarried a woman named Sayo. Kaito formed an intensely close bond with his stepmother, starkly contrasting his strained relationship with his father. He fiercely resents his father's perceived neglect and disrespect toward Sayo, particularly Chikara's coldness regarding her infertility struggles and his extramarital affairs with a younger woman.

Convinced he alone can offer Sayo the care and affection she lacks, Kaito's feelings for his stepmother shift beyond familial love into romantic and sexual desire. He initiates a secret affair with her. This decision stems directly from his belief that he is better suited to meet her emotional needs and his protective urge to shield her from his father's mistreatment. His actions are portrayed as a consequence of these protective instincts and his profound disillusionment with his father's behavior.