Hundred Note: Coffin Robin

In order for the detective agency Coffin Robin to accept a case, it must be "intriguing." What interests great detective Inori Kosaka, a former criminal psychologist, is not the case itself, but the crime. Not the victim, but the culprit. What series of cause and effect drive him to commit the crime? Looking to answer that question what the sole factor that led the scholar Inori Kosaka to become a detective. ...Dead men tell no tales. Knowing this, the robin asks the coffin "Who killed you?"

(Source: Kodansha, translated)