Bungo Stray Dogs: BEAST

BEAST (BEAST -白の芥川、黒の敦-) is a light novel by Kafka Asagiri, released in limited form 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.

Summary
BEAST narrates an alternate universe, with Ryunosuke Akutagawa joining the Armed Detective Agency, and Atsushi Nakajima acting as a subordinate in the Port Mafia. Under the mentorship of Sakunosuke Oda, Akutagawa strives to confront the Port Mafia's Boss, Osamu Dazai, and free Gin, his younger sister, from the organisation. The mission leads him to an intense feud with the Port Mafia's "White Reaper", Atsushi Nakajima.

Key Differences

 * Akutagawa's personality, while still intense and ill-tempered, has not been bred to be as homicidal nor impatient. He's still largely himself, but exercises a great deal of self-control not seen in the main universe. This Akutagawa succeeded in killing the men that killed the orphans he interacted with, and refused Osamu Dazai's invitation to the Port Mafia. In the Armed Detective Agency, his main goal is to find and retrieve his younger sister from Port Mafia.
 * Atsushi gains a reputation in the mafia as one of its strongest members, known as the "White Reaper". He's forced to wear painful restraints to keep the tiger in check, and uses the tiger's form frequently to defeat and often kill his enemies. At his core, he is still a fearful, timid person, wracked with guilt and remorse over the tiger going out of control and killing an unknown researcher before entering the mafia.
 * Gin Akutagawa plays a much larger role than in the main universe. She still works in the Port Mafia, albeit as a secretary rather than as a commander of Black Lizard. She is far more talkative, and holds a grudge against Akutagawa for 'abandoning' her in the past.
 * Sakunosuke Oda survives the war against Mimic, as he wasn't present during its main events. He left the mafia two years prior to BEAST's events, and now works in the Armed Detective Agency, acting somewhat as a mentor for Akutagawa. The orphans from the Dragon's Head Rush also live with him, along with ten other children. Alive and well, he's in the process of publishing his first book. His friendship with Dazai is nonexistent, having never met Dazai, and Odasaku allows no friendship between them. This is the only universe he survives.
 * Osamu Dazai succeeds Ōgai Mori as the Port Mafia's boss. Dazai is much grimmer, although still holds his dark, sardonic sense of humour at the worst of times. Dazai made many enemies, but the Port Mafia's influence spans even greater than Mori's influence managed. Somehow, he managed to obtain The Book, and, thanks to No Longer Human, the Book's ability to merge and diverge realities does not affect him. This makes him largely similar to the main universe's Dazai, and they are essentially the same person living different universes. Protecting the Book is Dazai's main priority, aware it's the only universe Odasaku lives in and achieves his goal of becoming an author. As such, Dazai allows many hardships for himself for Odasaku's sake. In the end, however, the Book's knowledge and his own struggles overwhelms him, and after asking Atsushi to continue protecting the Book, he falls off the ledge of the main Port Mafia building.
 * Mimic killed Ōgai Mori, leaving him absent in BEAST's universe.