Sui Kayama trains as a makai priestess under Jean Pierre Polnareff while maintaining a close friendship with fellow demon hunters Mikoto and Takeru Shiratori. Her well-behaved demeanor reflects strong adherence to traditional morals, including the conviction that sexual intimacy belongs solely within marriage or a loving relationship. She harbors lifelong romantic feelings for Yamato Tachibana.

As part of the "Black Cat Brigade," she investigates supernatural threats. During a haunted school case, she reunites with Yamato and reveals she encountered Takeru shortly before Takeru's disappearance. On a resort island incident, she accompanies the Shiratori sisters to probe disappearances linked to demonic forces. Her combat role as a "Squishy Wizard" denotes lower physical resilience than other fighters.

Her experiences include significant trauma across media continuities. Non-canonical OVA adaptations depict her enduring sexual enslavement by demons, with scenarios where repeated violations mentally break her into accepting a "Happiness in Slavery" state. These OVAs occupy separate timelines where events "Snap Back" to normal in later installments, erasing prior consequences. Merchandise depicts her in victimizing scenarios like "Sexual Slave" or "Female Dog" versions, representing alternative interpretations rather than canonical outcomes.