Bungo Stray Dogs: Dazai, Chūya, Age Fifteen

Fifteen is a light novel written by Kafka Asagiri, exclusively published to those who attend the Dead Apple movie in Japan. it details the first meeting of Chuuya Nakahara and Osamu Dazai, more about Mori's ascension as Port Mafia's new Boss and Chuya's background.

Plot
The ghost of the previous Boss haunts the Port Mafia. Dazai, who is trying to uncover the mystery per the Boss' orders, bumped into a young boy, who later revealed to be someone from the Sheep, an organization which recruits member of teenage ages. Who orchestrated the events behind the haunting? And does all these events have a link to Chuya's past?

Post-Conflict

 * Chuuya joins Port Mafia and becomes their new Executive in a short time.