99-banme no Kyuuketsuhime

Lores are substantial forms born out of urban legends. High school student Tsukumo Jin accidentally releases the strongest and most well-known "lore of the vampire," the butler (and lover) of Sylvia, the vampire princess who has taken on the form of a little girl.

To save his beloved little sister Shigure, Jin is tasked with banishing the "one-man hide-and-seek" urban legend with the help of mysterious transfer student Ayase. This is the beginning of the battle with "the last vampire," the hidden entity orchestrating these incidents.

(Source: NovelUpdates)