Yuki Nee-chan no Kannou Gokko.

Shin Yuki, a first-year high school student, stumbles upon an unexpected and compromising scene when he walks into an empty classroom. There, he finds his naked childhood friend, third-year student Yukino Kurose, passionately acting out a scene from a sensual novel using a mannequin as her partner. Ever since they were children, Yukino has harbored a unique hobby of recreating stories from books to truly understand how the characters felt, always relying on Yuki's assistance to bring her imaginative play to life. However, her latest choice of reading material has escalated their innocent childhood games into something far more intimate. Dissatisfied with the limitations of a lifeless mannequin, Yukino asks Yuki to step in and help her reenact the erotic novel just as he used to do when they were younger. She makes it clear that if he refuses, she will simply have to find another willing participant to take his place. Faced with this provocative ultimatum, Yuki is forced into a quick deliberation. Ultimately, the thought of another person seeing his older childhood friend in such a vulnerable state proves too much for him to bear. Driven by a fierce desire to keep her exposed form hidden from anyone else's eyes, he decides to accept her request, stepping into a deeply personal and sensual role-play that blurs the boundaries of their long-standing relationship.