Fūka "Saipū" Saitō, a 14-year-old second-year middle schooler, is defined by her striking physicality and unwavering romantic pursuit of her older brother, Hikaru. The nickname "Saipū" stems from her Cyclops-inspired hairstyle obscuring her left eye, paired with a statuesque 180 cm frame and curvaceous figure that command attention. Though once shorter than Hikaru, her rapid physical transformation over two years amplified social tensions, resulting in frequent awkward encounters fueled by overt romantic declarations and possessive behavior.

Despite her mature appearance, she retains childlike devotion to Hikaru, blending playful teasing with calculated provocations to unsettle potential rivals. Academically gifted and domestically skilled, she cultivates a polished image among peers. Her social circle features Rin, a mischief-prone friend whose bawdy quips she deflects; Nacchan, the deadpan foil to Rin’s antics; an infatuated class representative; and Kamiji, a tall, reserved peer grappling with hidden anxieties.

Interactions with Hikaru reveal layered strategies: she strategically cosplays as his favorite fictional heroines and crafts double-edged dialogues oscillating between flirtation and innocent wordplay. Questions about her concealed eye prompt cryptic warnings about containing "restless dark energies." Familial dynamics further complicate her quest—parents humor her brother complex, while cousins Akari and Souta Haruno intermittently plot to fracture the siblings’ bond. External threats like Eri, the charming president of Hikaru’s college radio club, exacerbate her vigilant jealousy.

All narrative iterations reinforce her singular focus on fulfilling a childhood marriage pact, leveraging physical dominance over Hikaru for comedic contrast against socially precarious scenarios. No deviations occur in adapted materials, maintaining her relentless pursuit as both driving force and source of situational humor.