Ayane is the younger sister and the primary catalyst for the show's chaotic events. She is characterized by a fiery, energetic, and highly reactive personality. Unlike her composed older sister, Ayane wears her emotions on her sleeve and is quick to anger, lash out, or act on pure impulse. She possesses a sharp tongue and a stubborn streak, often adopting a defensive, Tsundere-like attitude to mask her vulnerability.Her life changes when she and Kotone move into the small apartment of their childhood friend, Kazushi Sudou, to save money during college. The central plot kicks off entirely due to Ayane's impulsive desire for petty revenge. Believing Kazushi bought a cheap, low-quality shampoo for them on purpose, she decides to sneak into the bath while he is using it to exact payback. This reckless boundary-crossing completely shatters the platonic nature of their childhood friendship.Despite her aggressive exterior and constant bickering with Kazushi, Ayane harbors deep, long-standing romantic feelings for him. Her chaotic behavior is frequently fueled by intense jealousy whenever she perceives him getting too close to Kotone. This creates a volatile cycle where her pride prevents her from being honest about her feelings, yet her possessiveness drives her to demand his undivided attention. Height & Build: She stands at 159 cm tall and features a voluptuous, well-endowed physique with an F-cup bust size. She values her appearance highly, heavily prioritizing skincare and body care routines.Hair & Eyes: She has long, vibrant brown hair that flows down toward her waist. She frequently styles her hair tied back into a high, bouncing ponytail with sweeping bangs framing her face. Her large, expressive eyes are a matching brown tone, switching rapidly between sharp glares when angry and soft looks when flustered.Attire: Her standard outfit throughout the series is her school uniform, consisting of a sailor-style top, a short pleated skirt, and high socks. Off-duty, she favors casual, youthful clothing that is easy to move around in.

Kotone is the older sister and serves as the mature, grounded foil to Ayane’s chaotic energy. She embodies the traditional "ideal older sister" archetype, possessing a gentle, nurturing, and soft-spoken demeanor. Kotone is naturally polite and strives to maintain harmony, frequently acting as the voice of reason and the peacekeeper when Ayane throws tantrums or sparks conflicts in the household.Despite her maturity, Kotone's fatal flaw is her passive nature and her extreme difficulty establishing firm boundaries or saying "no." While she initially tries to keep their living situation decent and proper, she gets inadvertently dragged into the scandalous dynamic initiated by her sister. Once the boundaries between the childhood friends are blurred, Kotone’s polite resistance quickly crumbles under pressure.Beneath her calm, responsible facade lies a deeply flustered and surprisingly passionate individual. As she gets caught up in the same intimate and compromising situations as Ayane, she struggles with immense guilt over betraying her sister. However, her own deeply repressed affection for Kazushi ultimately surfaces, leading her to secretly indulge in the forbidden living arrangement while trying to maintain the illusion of the innocent older sister. Height & Build: She stands a bit shorter than her sister at 153 cm. Despite her shorter height, she possesses an even more exaggerated, curvaceous figure with a G-cup (or H-cup) bust, a 58 cm waist, and 85 cm hips. A running joke in her design is her worry that her clothes are rapidly getting too tight for her.Hair & Eyes: Kotone features a neat, bobbed black haircut that cleanly reaches her neck with straight-cut bangs. She has large, deep brown eyes that usually give her a gentle, laid-back, and innocent look.Attire: She is most recognized in her standard school uniform, which features a crisp shirt, a pleated miniskirt, and prominent thigh-high socks that emphasize her look.