Tsugaru Shinuchi

Tsugaru is a half-human, half-demon young man and Aya's assistant. He used to work at a freak show as an "Oni Hunter" (aka Demon Killer). He has an aloof personality and calls Aya his master, and the two occasionally engage in lighthearted conversation. He has the characteristics of a "demon" and is highly skilled in combat. In order to prevent him from completely turning into a demon, he regularly ingests the saliva of Aya, who is "immortal," to halt his progress.

(Source: Undead Girl Murder Farce Fandom Wiki)