Yū Nagami is a first-year high school student determined to become a published light novel author but facing repeated competition rejections. Critics specifically fault his female characters for lacking appeal. He hails from a household with a strict father and a younger sister, Suzuka Nagami. Suzuka excels academically and serves as her middle school's student council president. Yū mistakenly interprets her consistently cold and rude behavior as animosity, a perception rooted in his own insecurities rather than truth. While he cares for Suzuka, he has never previously felt protective or affectionate towards her.

His situation shifts dramatically when Suzuka wins a prestigious light novel award with her manuscript "The Story of a Little Sister Who Loves Her Big Brother So Much It Causes Trouble," a romantic comedy centered on a sister's devotion. To shield her school reputation and avoid their father's disapproval, Suzuka convinces Yū to become her proxy author. He takes public credit for her work under the pen name Chikai Towano. Yū agrees to this deception only until he successfully publishes his own original novel.

As the public face of Towano Chikai, Yū navigates the light novel industry. Key figures include his unconventional editor, Reika Shinozaki, who employs provocative methods to "inspire" his writing; fellow author Mai Himuro, secretly his favorite writer Enryuu Homura, who becomes both his admirer and rival; and illustrator Ahegao W Peace, known for her sexually explicit artistic style. His role demands participation in promotional activities like fan events and radio interviews, often alongside Suzuka to maintain their charade.

Throughout this collaboration, Yū remains largely oblivious to Suzuka's romantic feelings. He routinely misinterprets her advances or confessions as research for her next novel. This extends to other female characters' attentions, including Mai's covert stalking and Ahegao W Peace's overtly sexual propositions. His naivety is evident during forced fan-service scenarios, like applying sunscreen to multiple female characters at a beach event or role-playing for "illustration reference," where he displays visible discomfort. Professionally, he leverages his position to learn from peers like Mai and editor Reika, gradually improving his craft while balancing part-time work.

Within the reported timeline, Yū's development remains incomplete. He gains industry experience and refines his writing skills but has yet to publish his own novel independently. His relationship with Suzuka evolves through closer collaboration, yet he persists in misreading her jealousy toward other women and her genuine affection as part of her authorial persona. The narrative concludes without resolving whether he recognizes Suzuka's feelings or attains his professional aspirations.