Yumi Torauma (also known as Yumi Toraba) is the daughter of Director Torauma, head of the powerful Torauma Industries funding the Tsukuba Institute's Vietnam expedition. Her father opposes her relationship with Subaru Marita, son of a wheelchair-bound professor, deeming it unsuitable. He pressures Yumi to marry his chosen successor and enforcer, Goyō Yasui.

Defying her father, Yumi continues seeing Subaru and accompanies him to Tokyo Airport to see him off. When Yasui arrives to forcibly remove her per her father's orders, Yumi resists but is overpowered and taken away as Subaru departs.

Later, Yumi learns of Subaru's close association with Lamina, the mysterious green-haired entity from Vietnam. Interpreting their interactions as romantic, intense jealousy leads Yumi to angrily break off her relationship with Subaru, dismissing his claims of purely scientific interest in Lamina.

Seeking a direct confrontation, Yumi journeys into the snowbound mountains near Mount Washidaka where Subaru and Lamina hide. Yasui intercepts her, declaring his intent to marry her and secure control of the Torauma empire. Yumi rejects him and flees deeper into the mountains. Yasui pursues, but their confrontation is interrupted by Zamba, the Terminator-like assassin targeting Lamina. Zamba kills Yasui, allowing Yumi to escape and reach the isolated dam and power station sheltering Subaru and Lamina.

Yumi survives the climactic battle at the power station, witnessing Lamina's transformation and direct fight with Zamba. She reunites with Subaru during this final conflict.