Shinobu covers her face with a mask primarily to prevent others from getting involved in her personal affairs.
Kuki Shinobu, a distinctive character within her world, consistently wears a face mask as a notable part of her appearance. This choice is a deliberate and practical measure on her part. Her main reason for obscuring her face is to establish a clear boundary, aiming to prevent others from delving into or interfering with her personal matters.
This pragmatic approach highlights her desire for privacy and autonomy. By maintaining a degree of anonymity, Shinobu ensures that her professional interactions and public life remain separate from her private life. Even though she maintains positive relationships, such as being on good terms with her sister Miyuki, her mask serves as both a physical and symbolic barrier, protecting her personal space from unwanted intrusion.