Reimei no Grimoire

In a time when demon possession was despised and executed. 17-year-old girl Olivia is a heretic exorcist who, while being an exorcist, has a demon living within her. A lost client visits the office run by her partner, the handsome young man (who is often naked), Ru-kun, and her familiar, the perverted fly (who is actually handsome...!?) Bessan.

(Source: Hakusensha, translated)

Note: Includes two extra chapters.