Yoriko Yoshino is the mother of Chiaki and Chinatsu Yoshino, characterized by short black hair swept to the left and brown eyes. She exhibits a blend of maternal concern and authority, emphasizing her children's well-being by pushing Chiaki to eat more vegetables and visit home regularly. In the OVA, she shifts from warmly welcoming Chiaki to intimidating him to prolong his stay.

Yoriko openly admires Yoshiyuki Hatori, complimenting his appearance and congratulating him on his promotion to second editor-in-chief at Marukawa Publishing. She contrasts this by criticizing Chiaki's career as a shōjo manga artist, questioning his life choices and urging him to quit to "find a nice girl." She repeatedly expresses her desire for Chiaki, Chinatsu, and Hatori to marry and start families, remaining unaware of Chiaki's relationship with Hatori or Chinatsu's feelings for Hatori.

This ignorance manifests when she proposes Hatori date Chinatsu during a family gathering, causing discomfort that leads to Chinatsu storming out and Chiaki departing early. Undeterred, Yoriko persists in her efforts—such as insisting Chiaki take vegetables after he leaves—reflecting traditional expectations and an unintentional dismissal of her children's personal realities.