It was quite a while ago that i read this story I know it is an incomplete because the author had passed away.
The story was about a orphaned teenaged assassin who had retired and now was attending high school. The biggest things i remember about it was the main character smoked "coffin nails" constantly. The book was up to the point they were traveling to Japan. I don't remember if that's where the last chapter left off or if they made it home.
Thanks for the help