Yuri Kashima, daughter of Aoi Kashima and older sister to Yuji Kashima, possesses striking blond, very long hair and red eyes. Her young-adult body is strong and fit, a result of rigorous training and high school sports competition, though Yuji mocks her figure as lacking during their frequent arguments. Despite constantly fighting with her brother over minor matters, Yuri cares deeply for him as a sister. Beneath this dynamic, she harbors a sexual attraction toward Yuji, disregarding cultural taboos and her school reputation. Her personality blends tsundere traits—often acting cold or insulting toward him—with genuine, loving devotion when admitting her true feelings. This internal conflict fuels feelings of inferiority toward her mother, Aoi, and sparks jealousy over Aoi's sexual relationship with Yuji. Yuri occasionally accepts their relationship, provided she remains part of Yuji's life and their mutual bond persists as both siblings and lovers.

Living with Aoi and Yuji in a household marked by her father's unexplained absence, Yuri's life shifts when Yuji and Aoi play the cursed board game "Sōkan Yūgi" (Incestuous Game). This game forces players into escalating sexual acts. Yuri discovers their encounter through a dream. Later, while masturbating in the bathroom and thinking of Yuji, she resolves to become closer to him and initiates a new game session. Acquired from a mysterious salesman, the game traps players in a room, magically compelling them to perform erotic actions based on drawn cards or landed spaces, overriding refusal. Yuri's participation extends the cycle of coerced incest.

Her role culminates in pregnancy, resulting in a child that is both her offspring and Yuji's, making her both mother and aunt. This outcome forms part of her narrative resolution across official media.