Koyomi Yoimachi (宵町 暦 Yoimachi Koyomi?) is a resident of the same dormitory Michiru resides in.
She is a very cheerful girl, but she also has a double personality. Unlike Koyomi's innocent and sweet exterior, her other personality, "Yomi", is without reserves and is usually fondling Michiru, despite the latter's unwillingness to do so. It appears that Michiru is the only person able to awaken this alter ego of Koyomi's, merely by kissing her or simply calling her by name.
Yomi also has a special ability called the "Dead Man's Tongue", which allows her to channel dead people's words.
In volume 8 of the manga, Yomi takes on the form of a white snake, and in volume 10, when Toho uses Koyomi's tongue to obtain the "Dead Man's Tongue" ability for an exorcist experiment Yomi uses her snake form as a new tongue. In volume 10, Koyomi awakens and finds herself with Shito and Toho. She threatens Toho and vows to protect Shito, who is in a daze. It is mentioned by Yomi and Michiru that Koyomi had fallen unconscious, and when Chika and the others come Koyomi is lying on the ground in a pool of blood at Shito's feet (currently possessed), her "Dead-Man's Tongue" having been bitten out of her mouth and eaten by "Lao Ye". Later in volume 11, Yomi takes over Koyomi's body as Koyomi herself is dead due to a sacrifice to bring Shito's soul back into him. In chapter 65, Yomi tells Michiru that she is actually a boy; this is hinted at in earlier chapters when Michiru learns that Yomi was made from the spirits of the slaughtered boys from the Yoitsuhara clan. There is also a point in which Koyomi dies in order to bring Shito back for he is the one Koyomi actually loves.
- While Yomi seems to dislike everyone other than Michiru, Koyomi seems to have crush on Sotetsu, althought this is not really true. Koyomi feels nervous around him because of a previous incident that happened after getting drunk by drinking too much Fantan, when she woke up to find herself with Sotetsu naked in the same bed.
- Yomi was made from the spirits of the slaughtered boys from Yoitsuhara clan.
- Her voice actress is Kana Ueda, and Masumi Asano for her Drama CD appearance.