The Detective Is Already Dead, Vol. 1 by nigozyu

When the story begins without its hero

​Kimihiko Kimizuka has always been a magnet for trouble and intrigue. For as long as he can remember, he’s been stumbling across murder scenes or receiving mysterious attache cases to transport. When he met Siesta, a brilliant detective fighting a secret war against an organization of pseudohumans, he couldn’t resist the call to become her assistant and join her on an epic journey across the world. …Until a year ago, that is. Now he’s returned to a life that is normal and tepid by comparison, knowing the adventure must be over. After all, the detective is already dead.