Yayoi Amane serves as a shrine maiden at Kamaya City's Tenkai Shrine, performing spiritual purification and exorcism duties. She shares a close, long-standing friendship with Shingo and Shiori Sumeragi, parents of the series' protagonist. This deep connection makes her a trusted confidante for their daughter, Matoi Sumeragi, especially concerning Matoi's missing mother.

Her role extends beyond the shrine. Yayoi possesses extensive experience combating supernatural entities called "Nights." She formerly operated as part of an exorcist team, utilizing a contract with a higher-dimensional being—the Water Dragon God—to wield powerful water-based techniques in battle. However, prolonged combat has degraded this connection, making further use of her powers potentially life-threatening. Despite this limitation, she continues to offer tactical guidance against threats.

Functioning as a mentor, Yayoi provides instruction and support to younger exorcists, including Matoi Sumeragi and Yuma Kusanagi. Her guidance covers practical combat strategies and emotional counsel, informed by her firsthand understanding of the physical and psychological toll of exorcism. She emphasizes balancing spiritual responsibilities with personal well-being.

Physically, Yayoi has waist-length black hair and blue eyes. While her traditional shrine maiden attire often obscures her figure, her exorcist regalia and casual clothing reveal a buxom physique, occasionally noted in discussions about marital prospects. Her personality blends approachability and resilience, maintaining a "cool big sister" dynamic with the younger protagonists while acknowledging the risks inherent in her diminished combat capacity.