Atsushi as Arelcchino au
When Atsushi was a child he was placed into and orphanage aka the House of the Hearth. He grew up will all of his siblings but no one would really talk to him because of his curse, after he killed the previous Knave he became part of the Fatui. The cryo archon sent him on a mission to Yokohama (were Bsd is set) to investigate the armed detective agency, he cut his hair and put on some torn clothes to make it seem like he got kicked out of an orphanage and lied about his past. He run into Dazai and Kunikida with pure luck which started his investigation. Ranpo never found out what Atsushi was doing as her curse blocked him from finding out the truth. After Atsushi had gathered all the intale he needed he pretended to go missing when he was given his next task by the ADA. He returned to the Cryo archon with the information he had collected and then returned to Fontaine to visit her children.
This video too me a long time so I hope you enjoy :)