He is mysterious onmyouji of the Ashiya family who uses his abilities to summon shikigami for spying. His weapon is a longsword and he's apparently something of a womaniser.
Mutsuki has short gray hair, golden eyes, which he usually keeps shut, and small, oval shaped eyebrows. He uses a pink scarf to cover his mouth, and it is later revealed he has marks around it.
He is usually seen wearing the Yamainu Squad uniform. Other times he can be seen with a shrine outfit.
Mutsuki is mysterious and easygoing. He is loyal to the Yaimainu and always ready to follow Sousei's orders. Although he is flirtatious, he seems to have good intentions. Mutsuki is insightful and diligent.
When Mutsuki was little, his mother left their family. Apparently, she had no idea that the Ashiya family she married into could carry the Orochi's vessel. Mutsuki's mother's absence and hostility may have influenced the lack of warmth he feels from his household.
Taiko and Mutsuki had a comfortable relationship that resembled a parent and child's. For instance, Taiko would ask Mutsuki about his whereabouts, while the Ashiya family would not care.
Mutsuki respects Botan, and is very relaxed when he is with her. Botan also comforts Mutsuki, and wishes for him to live happily. Botan's kindness and affections remind Mutsuki of his mother. Thus, Mutsuki himself desires Botan to live a happy life.
- During Mutsuki's early Yamainu days, he admitted to spending every night at a different woman's house.
To Taiko: “But... If I was the Orochi, I would probably consume everything. My house and my family… If that happened, it would bother me. Whether alive or dead, I don’t want to cause trouble to anyone…”
To himself (if he were the Orochi’s vessel): “I would probably consume everything I cherish.”
To Kiiko: “Jeez… I can’t abide a blade raised against a lady, you know.”
To Hirari: "Spending the night in the same room as another man is torture."
To Hirari: "I may like women, but I dislike troublesome things. I especially have no interest in things that belong to other people."
To Botan: "I wish to remain as a mere observer."
To Botan: "If I become too absorbed in something, I'll end up losing sight of the things around me."