Mutsuki Ashiya, a 27-year-old male operative of the Yamainu Squad specializing in Orochi matters, was born on February 29. He stands 169 cm tall, weighs 60 kg, and possesses AB blood type. His appearance is marked by short gray hair, typically closed golden eyes, small oval-shaped eyebrows, and a signature pink scarf concealing tattooed marks beneath it. He is usually seen in his Yamainu Squad uniform or shrine attire.

Trained in traditional Onmyoudou, Mutsuki summons two female-presenting shikigami for reconnaissance and intelligence. Though proficient in combat with a longsword, he seldom employs magic during fights. The hidden tattoos under his scarf enable a specialized spell capable of shattering a person's will, utilized during interrogations.

Mutsuki blends mystery, flirtatiousness, and diligence. He projects a womanizing persona yet exhibits unwavering loyalty to the Yamainu Squad and follows orders from its leader, Abe no Sousei, without question. Despite their families' historical rivalry, he and Sousei collaborate effectively, demonstrating mutual respect and trust. This easygoing exterior masks keen perceptiveness regarding others' motivations and emotions.

His childhood involved familial neglect after his mother abandoned the household upon discovering the Ashiya lineage's connection to the Orochi's vessel, contributing to his emotional detachment from his biological family. He later formed a parental bond with Taiko Kumo, who showed him genuine concern. His relationship with Botan holds significance; he perceives her kindness as reminiscent of his mother and desires her happiness, yet maintains emotional distance to avoid personal absorption clouding his judgment.

Mutsuki harbors profound fears about the Orochi's influence, contemplating that becoming its vessel would mean he'd "consume everything I cherish." This dread fuels his reluctance to form deep attachments and preference for remaining an observer in conflicts, reflecting his underlying anxiety about causing harm.