Saeko Mizuki operates as Masumi Hayami's secretary, defined by her formal demeanor and professional efficiency. Her sharp observational skills and intuition enable her to detect Masumi's burgeoning romantic attachment to Maya Kitajima before he consciously recognizes it. This awareness drives her to persistently advocate for Masumi to disclose his secret identity as Maya’s anonymous benefactor, the "Purple Rose" who delivers purple roses. These interventions occasionally provoke reprimands from Masumi for exceeding her boundaries.

Physically, she stands 170 cm tall with black hair, black eyes, and an adult presence. Her surname "Mizuki" blends the characters for water (水) and castle (城), while her given name "Saeko" combines 冴—signifying clarity, serenity, coldness, or skill—and 子, meaning child.

Her responsibilities extend beyond secretarial work when she assumes the role of Maya Kitajima’s manager following Maya’s contract with Daito. In this capacity, she supervises facets of Maya’s professional engagements within the organization.