Hello World Light Novel

Naomi, a shy teenage boy, is visited by his future self and tasked with fixing his biggest regret--his classmate and soon-to-be girlfriend Ruri is going to be in a terrible accident. This "future self" knows all about Naomi's past, present, and future, and can tell him just what to do and say in every situation, from a first date to a deadly lightning strike. But does this mysterious man really just want to give Naomi and Ruri a happy ending, or is there something far bigger at stake?