The Shadow Princess (Bungo Stray Dogs Fanfic)

Kuroi Yanagi is a orphan girl with a mysterious past who is Atsushi's childhood friend and she is the only one who protects him from other orphans and caretakers that bullied him.

By the time they grow up, Atsushi was kicked out it of the orphanage but Kuroi refuses to let wander all alone so she left the orphanage with him. While the two wander around to find money and starving to death. That is until they spotted and saved a young man from drowning in the river much to his dismay.

The strange man then introduced himself as Osamu Dazai along with his partner named Doppo Kunikida whom are members of the Armed Detective Agency. With both orphans part of the Detective group, their lives as detectives begins.....