Kyosuke Kosaka, an ordinary seventeen-year-old high school student, lives a mundane life with a
strained relationship with his younger sister, Kirino. Kirino is the exact opposite of him: she is academically brilliant, a successful teenage fashion model, and universally admired. However, Kyosuke discovers a dark, closely guarded secret that shatters this dynamic. Inside one of Kirino's discarded anime DVD cases lies an adult visual novel titled *Little Sister Can't Be This Cute*. Confronted, Kirino breaks down and confesses her hidden passion as an *otaku* obsessed with anime and adult games featuring moe tropes. Desperate for a confidant, she drags Kyosuke into her world, forcing him to become her "life counselor."Initially resistant, Kyosuke finds himself drawn into Kirino’s eccentric struggles as she navigates the dual pressures of maintaining her glamorous modeling persona while hiding her otaku hobbies. Kyosuke uses his easygoing nature to help her reconcile with alienated friends, such as the gloomy otaku Kuroneko and the wealthy, enthusiastic web-designer Ayase Aragaki, who initially despises Kyosuke due to a misunderstanding regarding Kirino’s well-being. As the siblings navigate conventions, maid cafes, and gaming tournaments, their bond grows significantly, shifting from mutual apathy to deep, protective affection.Complications arise when Kirino’s modeling career requires her to move to America for a temporary study program. During her absence, Kyosuke’s social circle expands to include other romantically interested peers, such as his childhood friend Manami Kamiyama, who represents the familiar, comforting normalcy of his past. Upon Kirino’s return, the emotional stakes escalate. Kirino begins exhibiting intense jealousy toward the girls in Kyosuke’s life, forcing both siblings to confront the unconventional, increasingly blurred boundaries of their feelings for one another.To resolve their lingering emotional turmoil, Kirino proposes a temporary, simulated marriage, allowing them to test the depth of their devotion without societal condemnation. During this intense period, they confront the heavy emotional weight of their taboo desires, family expectations, and the judgment of their peers. Ultimately, they choose to step back from the romantic precipice, recognizing the impossibility of a conventional future together. Instead, they agree to return to their status as
brother and sister, albeit with a renewed, unbreakable bond. They promise to face whatever challenges await them hand-in-hand, having successfully navigated one of the most bizarre and emotionally turbulent sibling relationships imaginable, forever changed by their shared secret journey.