Brave Story Boku no Kioku to Negai

Synopsis

The 18 year old protagonist lives together with his dad and younger sister. His mother died when he was 13 in an accident and afterwords he had a sudden bout of amnesia and his relationship with his family deteriorated.

When packing to move out on his own he finds an old book from his middle school days that his mother gave him 'Brave Story'. Suddenly thinking that he can change his fate like the protagonist of the story he goes to an abandoned building in the middle of the night and he actually gets transported to the world inside the book, while also being de-aged to his 13 year old self.