Detective is Already Dead Light Novel Volume 1 (Mature)

'You will be my assistant.' During a plane hijacking four years ago, Kimitsuka Kimihiko wound up becoming the assistant to a beautiful detective named Siesta. The pair traveled the world, solving crimes and battling criminal organizations. Until Siesta died, anyway. Kimitsuka thought that would spell the end of his adventures and a return to ordinary life, but an encounter with a girl who bears an uncanny resemblance to Siesta suggests that fate has other plans.