Eriko Torii, known as Rosa Foetida during her Yamayuri Council tenure, stands as a prominent figure at Lillian Girls' Academy. As an only child from a wealthy background, she grew up surrounded by multiple protective older brothers—three or four, varying by account—which fostered her tendency toward boredom amid effortless academic excellence. Yet she exhibits intense, obsessive focus when intrigued, earning the "Snapping Turtle" nickname for her tenacity.

Her appearance features a prominent forehead, accentuated by a signature red headband and sparking playful rivalry with Sei Satou since their youth. She has brown eyes and short brown hair.

Within the Yamayuri Council, Eriko serves as grande sœur to Rei Hasekura, explicitly choosing Rei "to see if anything interesting would happen." This reflects her pursuit of stimulation through unconventional relationships. Dominant in their dynamic, she rarely faces successful challenges from Rei, as disagreements amplify Eriko’s resolve. She deliberately teases Yoshino Shimazu, Rei’s petite sœur, exploiting Yoshino’s jealousy over Rei’s bond.

Post-graduation, Eriko studies arts and harbors a noted aversion to dentists. She maintains an unrequited romantic interest in an older man fascinated by dinosaurs. Her eccentric personality includes posing complex literary questions to strangers, often beyond typical comprehension.

During the spring term in "Printemps," she prepares for graduation alongside senior council members, transitioning from her role. Her consistent traits—intellectual curiosity, selective intensity in relationships, and idiosyncratic pursuits—solidify her as an enigmatic, influential force within the academy’s social structure.