Saitō-san works part-time alongside the adult Jun Sakurada at their shared bookstore. An aspiring actress active in a local theatrical company, she often performs in stage plays with her brother. Standing 160cm tall with brown hair and brown eyes, she possesses a persistent and appreciative nature, frequently praising Jun's sewing skills and repeatedly inviting him to join her drama activities.

Her attempts to involve Jun result in a misunderstanding where her brother, approximately 195cm tall and three years her senior, mistakenly believes Jun arrived to help with manual labor for the troupe's stage backgrounds. Saitō-san expresses embarrassment over this confusion, though Jun completes the tasks. This incident highlights her role in encouraging Jun's gradual social reconnection.

Even as Jun becomes entangled with supernatural events involving sentient dolls, Saitō-san remains a steady, supportive presence. She consistently demonstrates sympathy towards Jun's social anxiety and isolation, serving as his grounding connection to ordinary life amid the fantastical occurrences. Her character arc centers on fostering Jun's interpersonal growth through their workplace interactions and shared involvement in theatrical preparations. The narrative does not explore her family background, aspirations beyond acting, or life outside these contexts.