Highschool DxD: This World Isn't Easy

Waking up, his entire being had transmigrated to another world. The system he was bound to was nameless, mute, and never issued tasks, only a steadily increasing "energy bar." Fortunately, after transmigrating, he inexplicably gained the ability of absolute memory, preventing him from worrying about his future. Just as he embarked on the path of plagiarism, he moved, relocating to his single-mother aunt's house. His aunt had a only daughter named Kirigaya Suguha... Just as he accepted a semi-sci-fi worldview, his high school, which he had only attended for half a year, suddenly announced its closure and merger with Kuoh Academy, which had inexplicably appeared in their town... His front desk neighbor was a white-haired, silent l*li, which he could accept. His next-door neighbor was Asuna, which also made sense. But what was with his little sister named Yuuki Mikan? And her sister seemed to have a good friend surnamed Yoshimura? His homeroom teacher was named Kyoudo, and she had a daughter who had just been admitted to Todai? His history teacher...