Seiyaku no Ephemera: Akuma Hakushaku to Honoo no Otome

In this city of Rosetta, where magic dwells in secret, Lizelle, who had no memories from when she was very young, was saved from a monster by Count Albert, a young man who manipulates magical flames. In exchange for protecting the girl, who was an "Ephemera"—a wonder drug that will bestow great power to demons, Albert demanded her to assist in investigating a certain incident he was following. Furthermore, she was ordered to work as a maid in the Count's residence...?! This encounter with a demon Count, weary of his longevity, will change the life of the girl forever. A romance fantasy set in the city of hazy mist.

(Source: Ichijinsha, translated)