Yayoi Mizuno is a biology teacher engaged to her colleague, Tatsuya Kubo. She possesses light purple hair, purple eyes, and a notably voluptuous figure with an H-cup bust. She organizes a camping trip for her student fan club, serving as both a vacation and her going-away party. Accompanied by Tatsuya, the trip is disrupted by a severe storm destroying their campground, forcing them into a nearby forest for shelter. There, they encounter a carnivorous plant-like demon called the Garanta, which attacks and sexually tortures the group for cross-breeding.

Prior to these events, Yayoi shares intimate moments with Tatsuya. Later, she discovers him engaged in a sexual encounter with a student, Kyoko. This coincides with unspecified internal changes within Yayoi, associated with the unfolding supernatural horrors. The narrative notes her transformation or possession as part of the horror, culminating in unspeakable acts in the series finale. Her background includes her teaching career and relationship leading to the engagement with Tatsuya before the camping incident.