Koyomi Yoimachi shares a dormitory with Michiru Kita. She outwardly displays a cheerful and innocent demeanor but harbors a distinct alternate personality named Yomi. This alter ego engages in unrestrained behavior, particularly making unwanted physical advances toward Michiru whenever Michiru kisses Koyomi or addresses her by name.

Yomi wields a supernatural ability called the "Dead Man's Tongue," which channels and voices the words of deceased individuals, becoming pivotal in subsequent events. Koyomi exhibits nervousness around Sotetsu Aso stemming from a past incident where they awoke naked together after excessive drinking, causing others to misinterpret her discomfort as romantic interest. Her genuine romantic feelings are reserved for Shito Tachibana.

A significant transformation unfolds in the manga's later volumes. During an exorcism experiment performed by the Xu Fu Mafia on Shito, Lao Ye forcibly extracts and consumes Koyomi's "Dead Man's Tongue," gravely injuring her. In the aftermath of this trauma, Yomi manifests as a white serpent, replacing Koyomi's severed tongue.

Koyomi sacrifices her life to restore Shito's soul after his body is possessed, resulting in her death. With the original personality deceased, Yomi permanently assumes control of Koyomi's body. Yomi later discloses that she is not merely a personality but a distinct entity formed from the accumulated spirits of slaughtered boys from the Yoitsuhara clan, rendering Yomi fundamentally male despite inhabiting a female form.

This complete possession by Yomi marks the conclusion of Koyomi Yoimachi's character arc.