Eriko Torii holds the prominent position of Rosa Foetida (Yellow Rose) on the Yamayurikai student council at Lillian Girls' Academy, where she mentors Rei Hasekura as her grande soeur. She selected Rei as her petite soeur purely "to see if anything interesting would happen," reflecting her enigmatic and eccentric nature. Academically gifted and highly skilled in diverse areas, Eriko commands respect within the council while maintaining an unconventional reputation.

She hails from considerable wealth as an only child, though her "four older brothers" are protective cousins. This privileged life and consistent excellence in all things often breed boredom, driving her to seek unconventional stimulation. When captivated by a topic, she plunges into obsessive pursuits, earning the nickname "Snapping Turtle Eriko" for her relentless tenacity. This trait complicates interactions for others.

Eriko possesses short brown hair, brown eyes, and a notably large forehead, typically accentuated by a red headband. This forehead became a point of childhood contention when Sei Satou mockingly dubbed her "Dekochin," igniting a lasting rivalry.

Her dynamic with Rei Hasekura features dominant, whimsical guidance. She frequently provokes debates with Rei, inevitably prevailing, leading Rei to warn others that arguing only fuels Eriko's stubbornness. Eriko also finds amusement in teasing Yoshino Shimazu, Rei's petite soeur, deliberately exploiting Yoshino's jealousy over Rei's bond with her.

Beyond the Yamayurikai, Eriko developed an unrequited infatuation with a male teacher, her persistence undimmed by the lack of reciprocity. She harbors a significant fear of dental visits.

After graduating from Lillian, Eriko pursued higher education in the Arts, aligning with her intellectually curious and creatively inclined disposition.