Sukisugiru kara Kanojo Ijou no, Imouto to shite Aishite Kudasai.

Keita Majima considers himself nothing more than an ordinary high school student navigating the routine of everyday life. However, his world is turned upside down when he unexpectedly lands a part-time position at the very game development company he has always dreamed of working for. What initially seems like a stroke of incredible luck quickly reveals itself to be a highly unconventional employment opportunity that will test his adaptability in ways he never anticipated. Rather than being assigned standard industry tasks, Keita discovers that his specific role is to serve as a professional "big brother for hire." The studio’s ambitious goal is to craft the ultimate sister character for their upcoming project, and they believe that firsthand experience is the key to achieving perfection. To accomplish this, Keita must step into the role of a devoted older sibling, providing the developers with authentic inspiration drawn directly from his daily interactions. Bound by a secret contract that blurs the lines between his mundane reality and his bizarre new profession, Keita embarks on a journey filled with unexpected challenges and comedic misunderstandings. As he strives to fulfill his unusual duties while keeping his peculiar job hidden from those around him, this hilarious story explores the chaotic lengths one young man must go to in order to help bring a fictional masterpiece to life.