Mizuki Himeji, a second-year Fumizuki Academy student, occupies Class 2-F after illness forced her to miss the entrance exam, relegating the academically exceptional second-ranked scholar to the lowest-tier classroom. Chronic frailty and dust allergies exacerbate her challenges in the dilapidated environment. Her appearance features light pink hair secured by a rabbit-shaped clip—a childhood gift from Akihisa Yoshii—purple eyes, a 150 cm (4'11") frame, and a 60 kg (132 lbs) build noted as above-average in Japan. Western lolita-style armor and a scythe or sword adorn her summoned avatar, which occasionally warps into a mature Morrigan Aensland-esque form during ESB malfunctions, amplifying her physique and sparking embarrassment. Alternate avatars include school-uniformed and housewife-themed iterations.

Secretly harboring childhood-rooted affection for Akihisa, Mizuki initially misinterprets her feelings before evolving into assertiveness, marked by jealous outbursts toward rivals like Hideyoshi Kinoshita and ominous yandere-like reactions. She forges a tactical alliance with Minami Shimada to monitor Akihisa, blending rivalry with cooperation. Academic mastery contrasts starkly with lethally inept cooking skills; her nutrient-overloaded dishes surpass aqua regia’s toxicity, incapacitating even resilient constitutions.

Her summon embodies scholastic dominance through heavy armor and broadsword prowess. A pivotal shift from formal "Yoshii-kun" to intimate "Akihisa-kun" underscores her emotional growth, culminating in a strategic kiss to deter his participation in an exam battle during his illness. Interactions oscillate between protective instincts, academic ambition, and guileless competitiveness.

Trivia reveals her adopted surname "Himeji" (princess road), an emotive rabbit hairpin synced to her moods, a shared birthday with Shouko Kirishima, alcohol sensitivity prompting reckless behavior, and a physique influenced by hereditary factors.