Suzuka Nagami is a third-year middle school student and the academically excellent student council president at Shirozakura Academy for Girls, known for her prowess in sports and household chores. She maintains a flawless, high-class image and possesses purple eyes with matching waist-length purple hair, characterized as a beautiful teenager.

Secretly, she writes a light novel titled "The Story of a Little Sister Who Loves Her Big Brother So Much It Causes Trouble," which wins a prestigious award. Bound by school rules against student employment and fearing her strict father's disapproval, she convinces her older brother, Yuu Nagami, to pose as the author under the pen name Chikai Towano, establishing their collaborative proxy arrangement.

Outwardly cold and aloof towards Yuu, Suzuka harbors intense, unrequited romantic feelings for him. These feelings emerge covertly, often through jealousy when other women interact with him. The premise of her novel directly mirrors these hidden desires, though Yuu consistently misinterprets her confessions and affectionate gestures as research for her writing, defining her role as a Covert Pervert and Clingy Jealous Girl.

Her character shows gradual shifts from restraint towards more overt expressions of affection. She experiences A-Cup Angst when comparing herself to bustier acquaintances. Animal Motifs occasionally appear, such as involuntarily uttering cat-like sounds like "nya" when flustered. After graduating middle school, she enrolls in Yuu's high school, increasing their interactions. Her jealousy escalates in response to his proximity to other women, including illustrators and fellow authors, prompting her to devise scenarios to monopolize his attention.

Key incidents highlighting her development include using a publisher's request for S&M roleplay to physically and verbally vent her frustrations at Yuu. During a beach episode, she reluctantly participates in fan service, including an intimate sunscreen application scene that showcases her discomfort and possessiveness. Her attempts to escalate romantic tension, such as requesting "research-based" dates, are repeatedly thwarted by Yuu's obliviousness.

Her academic and authorial achievements mark her as an Ace, contrasting with Yuu's perceived mediocrity. Despite her perfection, her inability to authentically convey her true feelings remains a central conflict, perpetuating a cycle of misunderstood advances and emotional suppression.