Binetsu Kakumei

Tatsuki and Oota are two beings from a supernatural, mythical realm, masquerading as "cousins" in the human world. Tatsuki is in fact the successor to Ryuu-Ou, the dragon king, and possesses the power of the Nyoi-Houshu, a magical dagger. Oota is a Zashiki Warashi, a protective house-spirit without a home, and is also Tatsuki's subservient lover. When their human classmate, Kouhei Misaki, accidentally witnesses the two having sex, he mistakenly believes that Tatsuki is forcing the quiet Oota into it, and he resolves to break the two apart.

(Source: Juné)