Sayaka Dejima is the personal maid assigned to Aria Shichijō within the Shichijō household. She presents a consistently professional and composed exterior, speaking with extreme politeness and control, while demonstrating high competence in her duties. Her appearance features long black hair, typically tied up, and dark eyes; she wears a standard maid uniform but changes into appropriate attire like hiking clothes for outdoor activities, styling her hair in a ponytail during such times. Despite her reliability, she harbors a deeply obsessive and inappropriate affection for Aria, sometimes watching her sleep at night—behavior that disturbs others but which Aria accepts without concern. This fixation manifests in fetishistic behaviors such as collecting and sniffing Aria's worn laundry and expressing eagerness to undress her. She also holds the key to Aria's chastity belt, managing access for necessities like using the bathroom.

Her personality shifts dramatically when overexcited, revealing immaturity comparable to Naruko Yokoshima. She is openly sexually experienced, with specific expertise in bondage and dominance, demonstrated when she restrained Takatoshi Tsuda after he accidentally encountered Aria showering. While she occasionally hints at casual sexual interest in Takatoshi, this appears as fleeting attraction rather than romantic pursuit. Her skills extend well beyond standard maid duties: she excels at cooking, cleaning, driving (including customized limousines), skiing, swimming, and identifying edible wild plants. She also cites proficiency in various sexual activities, aligning with the series' comedic tone.

In OVAs and later media, her actions emphasize her fetishization of Aria, including preparing baths with suggestive remarks about "golden showers" and later expressing intent to drink Aria's bathwater, prompting annoyed reactions from Aria. Her backstory notes she was hired shortly after Aria entered her second year of high school and reports to the household's head butler, Hashitaka. The kanji in her surname (出島) translates to "exit/protrude" and "island".