Yoko Sakakibara, a wealthy classmate of Azusa Noyama, acts as her primary romantic rival, driven by mutual affection for Yūnosuke Ogasawara. Her family's high social standing and her mother's friendship with Yūnosuke's family foster her belief in an entitlement to a relationship with him, prompting active efforts to undermine Azusa's connection. She employs manipulative tactics, including posing as a fortune teller to intimidate Azusa and spreading rumors to create distance between Azusa and Yūnosuke. Persistent, clingy jealousy defines her interactions, especially when sensing closeness between them.

Beyond this rivalry, Yoko engages in minor subplots. A sixth-grade student, after relinquishing his crush on Azusa upon acknowledging her affections lie elsewhere, shifts attention to Yoko, leading to a brief relationship. Separately, Azusa's younger brother Daizu experiences a short-lived infatuation with Yoko before moving on. Rumors among peers suggest possible academic challenges, such as not passing primary school, though details remain vague.

Her character shows minimal development across the series, with consistent failures in winning Yūnosuke's affection. She remains defined by her rivalry with Azusa and unrequited feelings for Yūnosuke throughout the elementary school timeline.