Kyosuke Kosaka, a normal 17-year-old high school student living in Chiba,has not got along with his younger sister Kirino in years. For longer than he can remember, Kirino has ignored his comings and goings and looked at him with spurning eyes. It seemed as if the relationship between Kyōsuke and his sister, now fourteen, would continue this way forever. One day however, Kyosuke finds a DVD case of a magical girl anime which had fallen in his house's entrance way. To Kyosuke's surprise, he finds a hidden eroge (an adult game) inside the case and he soon learns that both the DVD and the game belong to Kirino. That night, Kirino brings Kyosuke to her room and reveals herself to be an otaku with an extensive collection of moe anime and younger sister-themed eroge she has been collecting in secret. Kyosuke quickly becomes Kirino's confidant for her secret hobby.
Kyosuke is the 17-year-old main character of Oreimo. He has a distant relationship with his sister, feeling that this will never change. However, this suddenly changes when he finds one of her eroge and learns that she is an otaku. Through this, he tries to be a supportive older brother while realizing he himself knew very little about his sister, such as being almost flawless at school to working as a model. Later on, he takes up more of his role as an older brother to her, and appears to be able to tell what she is thinking or really meaning to say despite her tsundere personality. Despite this, she is generally the dominant person in their relationship, with Kyosuke being shown to be openly intimidated by her, only standing up to her in truly serious situations. Kyosuke prefers to have a relatively peaceful and simple life despite the fact that the drama Kirino brings makes his life anything but peaceful.Kyosuke Cosplay！