Kono Kanojo wa Fiction desu.

The story follows a naive young boy who has fallen deeply in love with an older girl widely regarded as a genius writer. His infatuation is so consuming that he has completely neglected his most guarded secret: for the past ten years, he has been meticulously crafting the character profile of a single fictional girl. Every detail of her existence—her birthdate, personality, hobbies, and imagined conversations—is recorded across a massive collection of notebooks. She has served as his closest companion and romantic ideal, but now that his heart belongs to someone real, he resolves to leave her behind forever. Determined to properly let go, he carries the notebooks to a nearby shrine with the intention of burning them in a symbolic act of purification. To his surprise, the upperclassman he loves happens to be there as well. As rain begins to fall, they strike up a conversation, during which she confesses that her lack of real-world experience has left her struggling to write her latest novel. Emboldened by the intensifying storm and her unexpected vulnerability, the boy decides it is finally time to reveal his own deepest secret. Filled with courage, he opens the bag containing his life's work—only to discover that the notebooks have vanished. In their place stands the very girl he spent a decade creating, entirely unclothed and suddenly, impossibly real.