BSD

Spinoff of Bungou Stray Dogs.https://web-ace.jp/youngaceup/contents/1000002/

In the world of Bungo Stray Dogs, Ryuunosuke Akutagawa is infamous for being one of the Port Mafia’s most dangerous members. But what if his past had turned out differently...? When Dazai decides to pass on taking in an orphaned Akutagawa, the boy winds up being recruited by none other than the Armed Detective Agency! With his sister still in the Mafia’s clutches, Akutagawa will stop at nothing to take her back. However, standing in his way is the Port Mafia’s top enforcer—the White Reaper, Atsushi Nakajima. As darkness and light face off in a twisted mirror of what could have been, a new tale begins to take shape…

a light novel focusing heavily on Chuuya Nakahara, where a mysterious man named Paul Verlaine, claiming to be Chuuya's older brother, appears and threatens to destroy everything important to him, forcing Chuuya to team up with a European A.I. investigator named Adam to stop Verlaine before chaos engulfs Yokohama.