Saki Mizuhara is a teenage girl residing with her older brother, Shuuji Mizuhara. She possesses long hair variably depicted as pink, orange, or yellow-blonde, typically arranged in waist-length twintails, and her eyes are red or garnet. Her attire consistently features a school uniform: a shirt, skirt, bowtie, over-the-knee socks, and occasionally a hairclip.

Though not biologically related—Shuuji being adopted by her family—Saki initially presents as a serious, honest student while secretly nurturing romantic feelings for him. These affections become mutual, leading to a romantic relationship after mutual confessions.

Their bond fractures when an anonymous stalker targets Saki with blackmail emails containing compromising photographs, such as images of her changing clothes or masturbating. The stalker demands increasingly public and immoral acts: masturbating in theaters, trains, and cafes, followed by demands for sexual encounters. Initially resistant and disgusted, Saki complies out of terror that the stalker will expose the photos to Shuuji or the public.

Saki's psychological state unravels. She withdraws from Shuuji, evading his inquiries and concealing her torment. Her initial revulsion warps into dependency on the stalker's demands, fostering an addiction to public sexual humiliation. Her behavior escalates to performing acts in progressively exposed settings, culminating in a stalker-orchestrated gang rape within a warehouse. Shuuji repeatedly attempts intervention but faces consistent rejection.

Her character arc traces a transformation from devoted sister and girlfriend into a victim of coercion and exploitation, defined by psychological collapse and physical violation.