Aisubeki Watashi no Kemono

Luca is a wizard who lives deep in the forest. He conjures a monster out of the darkness and gives it the name "Yug," without explaining why. Yug moves in with Luca, but in order to maintain his human form, Yug needs to make intimate contact with Luca. And that's when he feels as if Luca is seeing “someone else” through him…

(Source: Gentousha, translated)