Jishou Maou to Chuuni Cluster

When Kamishiro Kei was still very young, he went missing for 6 months and has no memory of what happened during that time. Now as a high school student, Kei believes that he is from the underworld and the heir to the demon lord throne. His childhood friends, twin sisters Noibara Yuzuki and Akezuki, are used to his delusional view of the world. They even play along in his fantasy as the servant of the soon to be king of the underworld.

(Source: MU)