Tantei wa Mou, Shindeiru.

Kimizuka Kimihiko is a crisis-magnet. From getting caught up in a crime scene to accidentally witnessing a drug deal, trouble seems to find him around every corner. So it is no surprise when his rather mundane flight suddenly enters a state of emergency with a dire need of a detective onboard. Unfortunately, his attempt at avoiding trouble is foiled by a beautiful girl with silver hair who goes by the codename Siesta. Declaring herself a detective, she unceremoniously drags Kimizuka into the case as her assistant. That incident spelled the beginning of an adventure around the globe that went beyond his wildest imagination. Putting their lives on the line, the two took down criminal organizations, prevented disasters, and saved thousands. But the curtain closed to their epic journey with Siesta's untimely death three years later. Resolving to live an ordinary high school life this time, Kimizuka spends a year maintaining a low profile. However, as fate would have it, a girl with an uncanny resemblance to Siesta comes crashing into his life, threatening to throw his peaceful days into disarray.