As the Ijuuin household's head maid, Mizuki Hanyuu holds authority over new butler Yuuto, enforcing strict discipline by reprimanding perceived negligence like tardiness or breaks. She possesses green hair and lilac eyes, consistently depicted in official sources.

After Yuuto survives an encounter with Serena—who typically drains partners' life energy—Mizuki develops deep investigative interest in this anomaly. She shares suspicions about Yuuto's resistance with household member Marie, and they actively pursue the truth behind his survival despite opposition from the family head. Her role shifts from strict supervisor to determined investigator seeking explanations for the supernatural event.