Akiko Himenokouji is the female lead and a central figure in her story. She is a sixteen-year-old second-year student at Saint Liliana Academy, where she serves as the secretary of the student council. In public, she presents the image of a perfect young lady, possessing a graceful appearance, polite manners, and an intellectual beauty, often accentuated by a pair of glasses she wears at school. Her physical features include waist-length jet-black hair and deep purple eyes.

Behind this composed exterior, however, lies her true and unrestrained personality. Akiko has an extreme brother complex, a fact she is entirely proud of and makes no effort to conceal. Her affection for her older brother, Akito, is the driving force of her life, leading her to launch frequent and bold romantic advances toward him. She is prone to vivid romantic daydreams about their future together and has a habit of reading forbidden-themed novels, especially those written by her brother.

Her intense devotion is rooted in her background. After their parents passed away, Akiko was adopted by different relatives and lived separately from her brother for six years. Their reunion caused her long-suppressed emotions to explode, and she moved to the city to be his constant companion. This separation has made her deeply dependent on Akito, and she believes that as long as there is love, ethical boundaries are of no concern, although she is initially unaware that they are not actually blood-related. Despite her assertiveness, her advances are often met with exasperation from Akito, who tries to maintain a normal sibling relationship, leading to a dynamic where her bold persistence clashes with his more reserved demeanor.

Within the story, her role as student council secretary often puts her in contact with other members. She is notably reluctant when Akito interacts with them, viewing them as potential rivals, especially Ginbei Sawatari, whom she considers her greatest competitor for her brother's affection. This jealousy fuels her protective and possessive behavior, ensuring that her brother remains the center of her world.

Akiko displays a notable contradiction in her abilities. While she is extremely diligent and persistent when it comes to pursuing her brother, she is almost entirely incompetent in other domestic areas, to the point of being unable to perform simple tasks like pulling weeds. Her days begin very early, as she wakes up at five in the morning, not out of a natural inclination, but for the reward of spending time with her brother. She actively seeks his affection, demanding praise and physical closeness, and uses any excuse to be near him, from waking him up in the morning to attempting to steal a kiss while he sleeps. Over time, her perspective shifts from outright hostility to a more strategic acceptance of the other people moving into their shared hostel, as she comes to see her bond with her brother as unshakable regardless of outside interference.