During school cleaning duties, Rurika Yamamoto accidentally broke a fossil. The male protagonist sharing her task took responsibility for the damage, prompting her to adopt a lifelong vow against lying.

Six years later as a high schooler working part-time at a convenience store, Rurika crossed paths with Kazumi, who entrusted her with a confession letter for Rurika's brother. Contacting Kazumi revealed her wish to date him before traveling to America for surgery. Moved by empathy, Rurika posed as her twin brother and accompanied Kazumi on the date, maintaining the deception throughout most of their time together.

Their date included an exhibit featuring a large sculpture resembling the fossil from her past. Seeing it made her drop her wallet in distress, nearly exposing her disguise. Kazumi later confessed she had recognized Rurika's impersonation but deeply valued the gesture. She clarified her trip to America was to pursue nursing studies, not due to illness. This encounter revealed Rurika's commitment to honesty alongside her willingness to help others through complex personal compromises.

Her appearance features a distinctive bob haircut in a 1920s-style and slight cute fangs in some depictions. The fossil incident remains a defining backstory element shaping her personality.