Sekisei inko manga

Sekisei Inko summary:

From MU: A young man, Kaneda, is inflicted with trauma-induced amnesia after discovering the body of his classmate and friend, Hiwako Shima. After being picked up from the hospital by a mysterious benefactor, he resumes school, and tries to piece together his memories of the incident. To make matters more confusing, his buried memories have manifested in the form of a surreal being, whose head is shaped like a crescent moon. Together, they set off to solve the mystery of Hiwako's murder.