Yuna Aiba is Makoto Aiba's stepsister, joining his family when her mother, Natsuki Aiba, marries Makoto's father, forming a blended household with Makoto and his biological sister, Misaki Aiba.

The pivotal event involving Yuna occurs when she, Makoto, Misaki, and Natsuki become trapped inside a hospital room. Yuna possesses a video game console acquired from a mysterious salesperson. She initiates gameplay with the group, triggering the room's immediate sealing, preventing escape or contact until the game concludes.

This video game operates under a curse similar to a prequel's board game. Landing on certain spaces forces players to draw cards demanding escalating sexual acts. The game's inherent curse enforces compliance. Yuna actively participates as one of the players subjected to these rules throughout the gameplay.

Her role centers on this forced participation in the cursed game alongside her stepbrother, stepsister, and mother. Possessing and starting the console game drives the core plot trapping these family members together.