Sui Kayama is a young Japanese woman with black hair and blue eyes, defined by her behavioral propriety and adherence to traditional moral principles, especially concerning intimacy within committed relationships. She lacks physical combat skills but possesses heightened spiritual sensitivity, enabling her to detect supernatural entities effectively during missions. This ability proves valuable for her team's demon-hunting operations despite her inability to defend herself.

This combat deficiency makes her dependent on protection, leading teammate Mikoto to prioritize safeguarding her during supernatural confrontations—a dynamic reflecting their close friendship and Sui's limitations in high-stress situations. Romantically, Sui harbors unexpressed feelings for teammate Yamato, marked by personal insecurity about his potential disinterest. Observing his prolonged attention toward her physique during a swimsuit incident triggers conflicting emotions of happiness and self-consciousness.

Her background includes training as a makai priestess under mentorship. Across official media, including OVAs and source visual novels, Sui maintains consistent characterization as a spiritually attuned but combat-incapable team member navigating interpersonal relationships amid monster-hunting operations, without significant documented development or backstory expansion beyond these established traits.