Bungo Stray Dogs: Dazai, Chūya, Age Fifteen

Dazai, Chūya, Fifteen Years Old (太宰、中也、十五歳) is a light novel written by Kafka Asagiri, exclusively released alongside the official DEAD APPLE premiere on 3 March, 2018. It will be included in first-print editions of DEAD APPLE's DVD and Blu-ray release on 28 November, 2018.

Plot
The ghost of the previous boss haunts the Port Mafia. At fifteen, Osamu Dazai, under the careful eye of Ōgai Mori, receives his first mission to investigate the matter. The investigation leads him to Chūya Nakahara, the infamous "King of the Sheep", a rival group of juveniles. With his own past unknown to him, and the lives of Sheep members on the line thanks to the Port Mafia, Chūya agrees to work alongside Dazai in order to unveil the truth behind the Port Mafia's former boss.